It is almost certainly down to the timing of when you got your credits for exploring. Remember that there was a day when FD doubled the rewards for exploring, in an attempt to balance somewhat against how relatively easy it was to gain credits through trading rather than exploring. Before that point, the consensus was that Elite exploring was at around 100 million and that only credits earned exploring was used as the ranking criteria (and that consensus could well have been wrong, of course). If all they did was double everything, the Elite requirement is now about 200 M.
So, if you had already earned 90 M on 'old money', that is 'worth' 180 M in 'new money'. So you would need another 20 M to get to Elite, and thus would get there at 110M (90M old + 20 M new). The same principal applies at any lower rank. Thus you can expect a significant variety in the numbers at which the level was reached, even if the limit is a simple static number.
I have around 40M earned before the doubling, at that was more than enough for Pioneer, though I do not recall exactly when I ranked up. I have not docked since, and am on a long mission to get to Elite. I estimate that I need another 120M or so in 'new money', and am between a third and half way there, assuming that my historic 'average earnings per system' translated into 'new money' is still correct.