It is equally possible that the person doing the injections doesn't know how to do math. The volumes used for injections are miniscule. When you are mucking with 0.01 and 0.001 it is an error of a factor of 10. May have given too much.
Good point. I viewed the package insert for Botox (available online). The practioner must first dilute the Botox with saline e.g. 1 ml of diluent yields 10 units Botox, 2 ml yields 5 units, 4 ml yields 2.5 units, etc. So yes, it is possible that it was either diluted incorrectly, or the wrong amount was given.
http://www.allergan.com/download/BotoxPI.pdf