bolger said:Because you might not win and lose every hand equally.
The odds for blackjack are 48% win 52% lose (around that) so if everything goes by the odds, you get £48.
If you use the blackjack strategy table for that casino, its just luck what you end up at. On one i ended up walking away with £20 of the bonus, on 888.com i walked away with 4xbonus ($250 i got up to when i hit the wager requirements, deposited $50, got $50 and played with that to wager).
The odds of blackjack are just odds, a football match can have odds of 100 to 1 but that doesnt mean thats whats goin to happen.
If you played an infinite number of blackjack hands, the odds would eventually even out but theres no guarantee when they'll even out. I played 400 hands and I had odds more like 28 to 72 so just be careful.