The two exp are not related at all.

Level up exp is gained when YOU water OTHERS and yourself

reward exp is gained when OTHERS or you water it.

So you could be gaining 0 level up exp a day if you don't do any watering yourself yet still claim the reward if there are enough watering the tree for you.

Also, level up exp is 1 exp per watering, always. Whereas the reward exp is tied to friendship level and you can get up to 3 for a single watering.

Edit: sometimes you can view the tree of others, are you sure it is YOUR tree that is lv 8?

Also, have you checked how much exp your tree have? hover your mouse over the level and it will tell you.