W H Y is the " flashback" scene at the end different from what they show in the beginning???

When afro-scott-franco and lemonparty-milk-penn are in bed and Franco says "youre 40 now, if you keep eating that cake youll be fat by the time your 50", Penn responds "If I make it to 50..." or summat.

Then in the " flashback" scene its like " youll be a fat-ass when youre 50" and penn responds "Ill never make it to 50"

Im sure its meant to be symbolic or some shit but it just doesnt make any sense.