I think it's probably that all of the bugs don't get discovered each beta. Between Beta 4 and Beta 5 there were major changes to the database structure, so that in itself probably generated a new wave of bugs. There are also occasions where fixing one thing will break something else, but that's probably not very common.

There were about 6 minor database changes between Beta 5 and Beta 6, so I think with beta 7 you will see a significant drop in the number of bugs, or at least a significant see more drop in the severity of them. I really think you'll see RC1 by mid to late October. After that, it really doesn't make any sense to get in a hurry just knock out something you can call a final release, since Boonex has stated that they'll be upgrade patches starting at RC1.