I guess 10 flights per booster is an average over, say, the whole time since the first booster successfully landed. In the list by this link there are ~70 boosters, and I don't think Falcons flew ~700 missions yet, so 10 flights per booster looks on average approximately correct, even though some - even quite a few - boosters flew significantly more that 10 times.

And recently, many payloads had their booster fly exactly once because the payload needed some extra 'oomph' to get into orbit so the booster was going to fast and to high to re-enter. When you are pricing for an average of 10x booster re-use you need boosters to go much more than 10x because one booster going 1x is going to bring your average down quickly.