I can't find this explained anywhere in the rules, forums, or google. Why does the Monk Table 3-10 show that at 6th level, Flurry of Blows is +4/+4/-1? A similar thing happens at level 11.
That's because the table includes the extra attack gained from having a BAB of +6 (which a flurrying monk gains at level 6, because he's treated as having a fighter's BAB while flurrying).
Similarly, at 11th-level the flurrying monk gains the additional extra attack from having a BAB of +11, and at 16th-level the flurrying monk gains the third extra attack from having a BAB of +16.
Ah, thank you. I didn't think that his Monk level literally changed his BAB while flurrying, but that makes sense now.