Thanks to all for the info.

In response to this post ...

I believe the builder was supposed to put in a 20-amp circuit and they
didn't. Easy to disguise with a 20-amp outlet. I put in the dual
outlet in order to run my compressor off of this "20-amp" outlet. So,
I'm not sure the builder is being completely honest about why there is
a 20-amp outlet on a 15-amp circuit, and whether or not that was really
supposed to be a 20-amp circuit.

I know a 20-amp circuit is not required for the sprinkler timer. Since
all customer add on outlets are supposed to be 20-amp, I think they
screwed up and don't want to acknowledge that I should have a 20-amp
circuit there.

No problem! I'm going to go look in some of their other homes under
construction and see whether the sprinkler circuit is 15 or 20 amps.

If it's supposed to be a 15-amp circuit, no big deal. I'll put in a
dual 15-amp GFCI outlet and move on.