Beginners like rapid because they aren't experienced enough to play fast yet.
So for rapid the ratings from 0 to 500 are beginners (for example).
But because most beginners don't play bullet, the rating 0 to 500 for bullet are already somewhat experienced... meaning 500-1000 bullet players are even more experienced than that.
So 1000 bullet > 1000 rapid
I think in addition to what llama said, blitz and bullet ratings tend to be lower for most people even above beginner level. For example, it's common for me to see like 1700 rapid and 1600 blitz, or 1600 rapid and 1400 bullet (some of my friends' stats.) The scales aren't calibrated to each other.
Yeah, it carries on up to higher ratings, so 1200 blitz > 1200 rapid, etc.
At some point it probably somewhat equalizes since e.g. titled players play blitz but not rapid... but in any case, IMO, ratings like 2400 rapid don't exist so it just sort of stops abruptly (they don't exist if you're playing random players)... please send me any accounts you find that you think are legit 2400 players (or higher) and I'd be happy to find out I'm wrong 
But obviously for blitz and bullet ratings go past 3000.
I had been playing bullet/blitz under the 1000 and i noticed that 800 somehow play better games in blitz and bullet then their rapid games
For example if i play against an 900 in bullet or blitz he will suck but he wont be completely terrible
On the other side if i play a 900 rated player in rapid he will somehow hang mate in 7 moves.