I always thought that the executive bonus is calculated the following way:

a. 25% of the credited award miles in case of fixed miles (e.g. 500 exec. bonus for an intra-EU flight in D, 125 for S/W that earn 500 miles etc.)
b. 25% of the distance miles in all other cases.

But there seem to be exceptions. This is the credit I received for a recent flight on UA:

SAN FRANCISCO-ONTARIO +250 +250
Executive Bonus +91 +91
UA5336/UA5336
Economy Class, V

I was expecting that rule a. from above would apply and consequently only 63 miles executive bonus. It took me a while to figure out why M&M credited 91 miles executive bonus: It is 25% of the distance miles, as SFO-ONT is 364 miles!
I guess this must be a rather rare case where the executive bonus based on the distance miles is higher than based on the minimum miles.
It is definitely the first time for me to receive a higher credit for a flight than I expected - although my gain was minimal ….