Look from the inside out, you'll see that your front windows are in fact darker than the rears. Throw a meter on them, and they will measure about 16-17% while the rears measure 23-24%. It will match better from the outside, but the actual matching tint would be closer to 30%.
My M3P has 30% all the way around on it, and the fronts appear lighter than the rears due to the amount of light that comes in from the windshield. That's pretty typical unless you tint the windshield to match the sides/rear.