General query about pricing approach

There are two different points, profit maximization and revenue maximization. These both are calculated in different way.
I am confused that why these point are not same? If a product have maximum revenue so it should have maximum profit at that point which is not found here. These both points are totally different.

For example,
A= 90
B = 0.006
MC= 20

For profit maximisation:
MR=MC so (a-2bq)=20 so q will be 5833 units.
For revenue maximisation
MR=0 so (a-2bq)=0 so q will be 7500 units.
Both units are different. I am not getting the concept.