Not sure about in the Simpsons universe, but there are definitely IRL humans with > 10 digits in their hands (polydactyly). But it’s very rare, so the mean still rounds to 10 (or 8 in the Simpsons), even when you factor in people who have fewer due to injuries or genetics.

Ok fair enough, I might go for mode, but I’m still not buying mean. In my, admittedly incomplete, study of this topic I’ve encountered many subjects with 9-10 fingers, but never any with more so my hypothesis is that the population mean is 9.something. More work needed.