Shyvana is the top jungler because all of her competitors have been nerfed over the last few patches. She's relatively easy to play and has a low-risk farm-heavy style while being a strong duelist that can easily survive invades and make invasion attempts of her own (whereas ganking is higher risk because a countergank can quickly result in an enemy double kill, and while it should be higher reward by snowballing an ally, Shyvana's fast clears allow her to snowball just as quickly by not ganking).
That all said, I'm fine with Shyvana as a top-tier jungler. I don't want that kind of champion on my team that often, nor do I mind playing against her at all. Sure, she's tanky and does lots of damage, but she has terrible engage for a tank and poor sticking power if allies peel for you.
EDIT:
Oh, and on Garen, not sure. His win rate is around 10th, which is pretty high for such a simple champion at that elo. However he hasn't been given enough credit since his rework (he's a bit more dynamic now, and the villain system is under-estimated) and parts of his kit are still skill-dependent (especially when to use the silence). He's performing at #1 because of a high-ish win rate alongside high play rates, good KDA, and good numbers overall. A good KDA is rather meaningless though as winning is all that matters - Garen SHOULD have a good KDA because he's insanely tanky and shouldn't die very often while still being capable of racking up kills - this doesn't translate in to winning games necessarily because like Shyv he has poor engage and sticking power, meaning he can't force that late-teamfight-ace to make a win happen before the enemy Vayne outscales him.