If we have P-R for present and P-R-ISH for presentation and R-P for represent and R-P-ISH for representation, why is it E-S-P-R for express BUT E-X-P-R-E-ISH for expression?

What's the rule for adding the final E?

This was one of those things I did automatically and then went back and checked. Yes, I did it right but then it seemed inconsistent. Help.