In some areas (e.g. law) I wouldn't expect there to be much difference in that time - in Scotland at least, 3.5 years after you graduate with a law degree you've only been qualified for 6 months! (The law degree is followed by a postgraduate course for a year then two years training before you qualify).
I'd be interested to see the results for 10 years after graduating!
Only a tiny percentage of law graduates go onto become lawyers (<10%), so it's probably doesn't have a significant impact on the overall data even for law degrees.
Certainly it's possible that in some careers that there is a divide, but due to the the careers not being common that it doesn't show up in the stats. But there's no evidence this is the case.
I'd be interested to see the results for 10 years after graduating!