People will advance at different rates, especially new racers. It's all based on average incidents per corner, and it takes over 1600 past corners into account.
I had a string there where I would only advance .08 or .10 per race, with a couple 1xs per race. But then I had 1 completely clean race, and my rating shot up .55.
So since it's an average, if you come out of the gate very safe with no incidents, you could be a D in just a couple races. I went to D in like 3 or 4 (I was very slow, so I let people by), then I started getting fast and racing people and I slowed right down in my progression because incidents naturally started to happen more often.
What I'm trying to say is it's hard to compare different individuals' safety rating, since everyone accrues incidents at different rates - some have a string of clean races and then one really disastrous one, and some people have a string of light incidents that add up over time. This affects your gains and losses in very different ways, especially across license levels.