You have to realize two things:
1) There are VASTLY reduced rewards for killing the same guy over and over if he doesn't get kills in between. If you kill him 10 times in a row, those last kills are going to net you like 50 gold apiece or something stupid like that.
2) Its extremely difficult to gank top when you are playing from bot. The only regular jungle spawns on that side are wolves, and they are still quite a ways from your lane. The jungler ganking top gives up quite a bit to go for that gank.
Or to put it another way, I got 8 of our teams 10 kills. The other jungler had 3 kills at the same time. Yet I was STILL behind on levels because I was top so much. I didn't get my first 5(!) reds because Shy was aggressively counter jungling and to be honest, I was top way too much as it was.
I can't consistently give away XP and gold to go for kills that are worth 100 gold if successful. The math just doesnt work out. Especially when other lanes are getting mauled in the process (Shy was sitting bottom, stealing my red, then ganking bottom a lot). If bot is struggling, its not really the best choices to abandon them and let them both feed to just get kills on somebody top who isn't worth much any more, especially since I have to abandon wraiths, red, and double golems to do it.
The other major factor is that I have to give up dragons to do this. This wasn't a big deal since they were bad, but in a real game with good players, if their jungler gets my red, ganks the bad bots successfully, then its a free dragon for them. I can't stop it any more. Plus I gave up half my jungle. Ugh.
Because of this, the basic rule is that top just can't expect many ganks. If he gets that many, the jungler gives up half their jungle to aggressive junglers, and risks giving up free dragons. Its much more viable for the jungler to gank mid and to gank bot, especially when playing from the bottom side when more of the jungle spawns are bot.