Also realize if this "in and out" ride is only 20 miles, it takes some people (me included) about 10+ miles to totally warm up. The first few miles I tend to spin, then gradually increase power. I don't often push harder until mile 10+, so it's difficult to compare power in this scenario. If I warmed up for 10 miles, then did the "in and out" ride it would get a better comparison. I'm too lazy at the moment to look at the power numbers for this type of ride, which I do often.

Regarding the windy vs. calm days, I agree that the aggregate speed during calm days is often higher, especially when riding solo. Oddly enough, it seems when doing group rides the aggregate speed during windy rides if often faster, since some of the stronger riders can push through the wind to help the full group, while most can haul ass in the downwind.