I can't be the only one that has noticed that these one week delays happens when there is a bundle of Ultimate comics coming out on the week it was originaly going to be released. (This may be because another Ultimate title was delayed and put on the same release that for exemple.) So Marvel choose to delay one of the titles so they get a more widespreed release of the Ultimate comics.
Why bother doing this? Because the majority of comicbook reader is still teenagers or younger and don't have that much money or is still dependent on their parents for money, so they can't afford more than a limited number of comics each week and if there is many titles released one week they might have to choose just a few of them and wait until they have enough money to buy the other titles, and then the interest of the comics not bought might have disappeared and they spend the money on something else.
But there is many comics released each week. Why don't delay another title not Ultimate instead? Because many comicbook readers today is just buying Ultimate OR 616 comics (for Marvel) and nothing else so delaying a 616 comic wouldn't make a difference on the Ultimate sale.