Also if you reset your town it resets your tree. So if you had 500 hours on your last town it will appear on your activity log but you're tree will still only be the size of how long you've played on your current town. Activity Log has showed since when you first started playing overall, not how long you've played on your current town.
From the pictures on the animal crossing wiki, it looks like you can tell growth stage by the thickness of the trunk. If your ingame date is set to the same as the current date then the leaves will still be growing back from winter, so even if you should have more clusters of leaves, there's not as many
I already do, play about an hour a day. I'm talking about the CLUMPS. The tree has only got 3 clumps despite being level 4 in stage (and the level 4 tree has got more than 3)