Why are albums rated higher at Amazon than at RateYourMusic.com?

I hang out a bit at both sites, and wonder why there are so many more 5 star reviews
at Amazon than at RYM. Well the obvious answer is “Fanboys pumping up the ratings
of their favorite artists”, but a high rating at RYM would presumably accomplish the
same end, right? IMNSHO a “Five” star album can only be a timeless classic: given
the ability to choose half stars (hmm Amazon doesn’t have half-stars but RYM does),
I’d probably rate only about 10-20% of the albums in my collection as 5 stars, and
if forced to round up or down from 4.5 I’d probably round down most of the time.

Comparing the most recent album, Deadwing, of my fave band, Porcupine Tree,
and looking at the 10 most recent reviews:

Amazon:

5
5
5
4
5
5
5
5
5
5

Overall 4.5 (which could mean 4.25-4.74)

RYM:

4.5
5
4.5
4.5
5
3.5
4
4
4.5
5

Overall 4.04

I’m just fascinated by the differences you find in otherwise similar communities…