Just wondering, why if I'm like...37% from one level goal, if I remove the goal then add it again, the % lowers? I was ~30% on my mining goal to 90, removed the goal, added the very same goal again and the % dropped, I know this doesn't affect my XP, I'm just curious about why did it happen.

It goes off the XP you're on and how much is left. so naturally the % will shrink from 100k/500k to 300k/500k

Yeah but...Say I was 900k from my goal and the % was 30%, cleared the goal and started it again, now I'm 900k from my goal but 15%

Since there's a difference in where your goal is relative to what xp you had before, it will change.

In that case, shouldn't it start from 0 again?

I'm not really sure why it doesn't start from 0. I think that it starts from where your last level was, maybe. I'm guessing.

Oddly enough, the % changes if I gained a level in the skill... Say I set a 90 mining goal at 86, and leveled to 87, if I restart the goal, the % changes but if I'm already 87 mining, and restart the goal the % stays the same, maybe it doesn't count the XP from the past level, so if I gained 50k XP after 87 mining, that will be the only XP to count for the new %? It's the only thing that makes sense to me.