It's certainly not going to explain everything but that's a place to start. Find out from Denon/Audyssey whether it is indeed 1/12 smoothing (or maybe someone on ASR knows definitively). Set the smoothing on REW to be the same. At least that aspect of your comparison will be apples to apples. After that ... great question, and I'm curious to know the answer, but way above my pay grade. The author / maintainer of REW (JohnPM) is on ASR; if he has any insights into how Audyssey measurements are done he could compare / contrast the two systems.
My only wild guess would be to say that the free mic included with a Denon AVR -- I'm assuming that's what you used when measuring via Audyssey -- has different capabilities than the UMK-1. How much worse (or better? probably worse) it is I couldn't say, but that might explain at least part of the difference.