There is another possible reason that a 15 amp GFCI was installed. The actual capacity of most "normal" outlets you buy is only 15 amps. If you have several on a circuit, it is assumed that they will not be loaded over 15 amps for any one outlet and other outlets will take some of the capacity of a 20 amp circuit, and the code allows that multiple 15 amp rated outlets can be used on a 20 amp breaker. However, if you only have one outlet on a 20 amp circuit it must be a heavier duty 20 amp rated outlet (usually you can identify them as they have a T slot on one side). Thus, if in a place like a home where there is a single outlet for the washer or washer and gas dryer, and that one outlet only is fed by a 20 amp circuit breaker, it must be a heavy duty outlet.
Back to the issue at hand. If the GFCI which AS installed in the panel is only serving one outlet (like in the bathroom) they may have put a 15 amp GFCI in rather than a 20 amp GFCI so they could use a regular (not heavy duty) outlet.
This is speculation only, but may be the reason for the 15 amp GFCI you have found.