(MOP at page 132): Jam-or-Fold NLH with stacks of 2000. SB posts 0.5 and BB posts 1. Calling range for BB is AA only.

To calculate the "equity" (which I read as EV) of SB jamming ATs, their approach is:

EV of jamming ATs

= p(BB folds)(pot) + p(BB calls)p(SB wins pot)(pot size) - cost of jam
= p(BB folds)(1.5) + p(BB calls)p(SB wins pot)(4000) - 2000

Working through MOP, my approach to these calculations has generally been:

EV of jamming ATs

= p(BB folds)(+1) + p(BB calls) x (p(SB wins pot)(+2000) + p(SB loses pot)(-2000))

So my reference point is stacks at start of the hand and the numbers I plug in are gain/loss relative to that.

I've liked this approach because it reflects the narrative in my head as I'm thinking about the game tree: "When SB jams, the BB will fold sometimes and SB will win his blind. When the BB does call, SB will gain 2000 some % of the time and lose 2000 the remaining % of the time ..." This approach has led me to correct answer for MOP games until now. But I'm just not arriving at the book's answer for this calculation.

My assumption was that both calculations were the same thing but just laid out differently.

What's my blind spot here?