It's certainly a tricky business rating rarety. To rate accurately you would need to know:
1) how many of each set were produced, and how many sold
2) how many of those sold have been destroyed, discarded or damaged beyond repair
3) How many have been laid aside MISB by collectors
4) How many have been disassembled for customisation
I think the ratings are a guesstimate, based on the fact that early year production runs were m uch lower volume, and the older a set, the more chance it has been destroyed. Also, collectors were rare in the early days, and much more common now, so few modern sets will ever be considered rare.
On top of that, are we talking about rarety of MISB sets, sets in good but used condition, or sets that could be reassembled if their owners cared to? I almost never keep a set as a "set", I just add it to my stock for that theme.
Looking at sets coming up on eBay is certainly one way to rate rarety (i.e. how easy is it to acquire a set NOW?), but doesn't account for people who don't sell on eBay.
The ratings must be intended to apply globally, but as Ras says, there are sets that are much harder (and some that are much easier) to acquire in North America than in Germany.
