It's because the combo count is so high. If you've ever tried making up some of your own combos with Viola, you'll experience the same thing. However, this doesn't necessarily mean 21 is the max amount of hits for a combo. Elysium can get 50+ lol.
Hey WhYYZ. Actually, I used A+B twice in another combo, B!E! didn't fail me, but that was on a stage with a breakable walls...I've even done A+B W! A+BW!, A+B, B!E! 4B...