It's ALL about money. Here is how:
1. If USCIS were to follow DOS's July Visa Bulletin, it would have opened a flood gate in July - and they would lose tons of extra money because of this; they would also get tons of extra WORK in one month. So between the following two choices, which one would you choose, if you were USCIS director:
A. Tons of extra work in one single month, for no extra money;
B. The same amount of work spread out over months if not years, with extra, extra money
Only a fool would choose A.
2. So intead of complying with DOS, USCIS decided to allocate all remaining visa numbers in June, forcing DOS to revise its visa bulletin;
3. The result: USCIS will get its cash cow going.