Here's why it seems to be a lottery, to me. The Canons of Dort (from which TULIP was formulated) says:
God’s Eternal Decree
The fact that some receive from God the gift of faith within time, and that others do not, stems from his eternal decree. For “all his works are known to God from eternity” (Acts 15:18; Eph. 1:11). In accordance with this decree God graciously softens the hearts, however hard, of the elect and inclines them to believe, but by a just judgement God leaves in their wickedness and hardness of heart those who have not been chosen. And in this especially is disclosed to us God’s act—unfathomable, and as merciful as it is just—of distinguishing between people equally lost. This is the well-known decree of election and reprobation revealed in God’s Word. The wicked, impure, and unstable distort this decree to their own ruin, but it provides holy and godly souls with comfort beyond words.
Election
Election is God’s unchangeable purpose by which he did the following:
Before the foundation of the world, by sheer grace, according to the free good pleasure of his will, God chose in Christ to salvation a definite number of particular people out of the entire human race, which had fallen by its own fault from its original innocence into sin and ruin. Those chosen were neither better nor more deserving than the others, but lay with them in the common misery. God did this in Christ, whom he also appointed from eternity to be the mediator, the head of all those chosen, and the foundation of their salvation.
And so God decreed to give to Christ those chosen for salvation, and to call and draw them effectively into Christ’s fellowship through the Word and Spirit. In other words, God decreed to grant them true faith in Christ, to justify them, to sanctify them, and finally, after powerfully preserving them in the fellowship of the Son, to glorify them.
God did all this in order to demonstrate his mercy, to the praise of the riches of God’s glorious grace.
As Scripture says, “God chose us in Christ, before the foundation of the world, so that we should be holy and blameless before him with love; he predestined us whom he adopted as his children through Jesus Christ, in himself, according to the good pleasure of his will, to the praise of his glorious grace, by which he freely made us pleasing to himself in his beloved” (Eph. 1:4-6). And elsewhere, “Those whom he predestined, he also called; and those whom he called, he also justified; and those whom he justified, he also glorified” (Rom. 8:30).
Election Not Based on Foreseen Faith
This same election took place, not on the basis of foreseen faith, of the obedience of faith, of holiness, or of any other good quality and disposition, as though it were based on a prerequisite cause or condition in the person to be chosen, but rather for the purpose of faith, of the obedience of faith, of holiness, and so on. Accordingly, election is the source of every saving good. Faith, holiness, and the other saving gifts, and at last eternal life itself, flow forth from election as its fruits and effects. As the apostle says, “He chose us” (not because we were, but) “so that we should be holy and blameless before him in love” (Eph. 1:4).
So, following from these assertions, Christ's atonement is limited to the elect.
The Saving Effectiveness of Christ’s Death
For it was the entirely free plan and very gracious will and intention of God the Father that the enlivening and saving effectiveness of his Son’s costly death should work itself out in all the elect, in order that God might grant justifying faith to them only and thereby lead them without fail to salvation. In other words, it was God’s will that Christ through the blood of the cross (by which he confirmed the new covenant) should effectively redeem from every people, tribe, nation, and language all those and only those who were chosen from eternity to salvation and given to him by the Father; that Christ should grant them faith (which, like the Holy Spirit’s other saving gifts, he acquired for them by his death). It was also God’s will that Christ should cleanse them by his blood from all their sins, both original and actual, whether committed before or after their coming to faith; that he should faithfully preserve them to the very end; and that he should finally present them to himself, a glorious people, without spot or wrinkle.
Piper new full well that some of those that would watch his video would be those whom (in his view) were not of the elect. Yet, he still tells them 'Christ died for your sins'.
He deceived them. He hasn't revealed to them the actual tenets of what he believes scripture teaches. For me, this is the crux of why Calvinism trips up - it comes against the very essence of the gospel itself.