I dunno.....that whole last page seemed like it was implying something suspicious....
Danzou even came to tsunade later and asked if she picked out a good anbu to lead the mission, then said "good, good" and walked away. Danzou seems to have alot more planned than we'd expect.

Also, I'm sure a few anbus are sneaky enough that they can switch sides without the hokage noticing. It helps that all they have to distinguish them is their mask. Yamato probably has 2.