NASCAR Racing: Numbers, personal preference, paint job

Question
Why do some of the numbers on cars (e.g. Tony Syewarts 20) lean towards the back of the car and some lean towards the front?

Answer
The faster the car, the more the wind blows it toward the back.
No, it's just personal preference. Whoever designs the paint job figures in which way to lean 'em. No deep dark secrets.
I will throw in that the numbers on the roof were ALWAYS placed to face the infield (rather than the stands) due to the old scoring done by "official scorers" in the scoring stand in the infield. Made it easier to tell which number to write down for what time on what lap.