Here are the posible reasons base on experienced and what I know
1. I think it is because of you amount that you want to transact
It's been going on for weeks now.
It is not just you who do a transaction
Lets say
We have 2 lanes in the cyber space
Line 1
Fee is 1USD
And you have to fall in line for that transfer
But for line 2 it is 2USD
And you have to fall in line too BUT it is much more faster than the fee of line one
In short the one who pays higher gets a transfer faster but not in one click because like I said many people do a transaction from time to time.