Basically, WGS 84 World Mercator is a conformal map projection, meaning it preserves shape (not area).
The UTM projection is limited to a small area with a defined maximum distortion, so the area calculation should have minimal distortion within the zone.
This is a very broad answer, and I am certain that others will expand on it.
Jim Cousins already addressed the main issue, i.e, Mercator projections distort area, especially the further you get from the equator.
I recommend reading Tissot's indicatrix helps illustrate map projection distortion and Tissot circles