Cant speak for xp of all players and ships - maybe inventory supply at stations work differently / randomly for different players, ship types, and/or faction rep - or some combo of these and othee factors.
But i'm allied with fed and empire, and while Diaguandri is excellent source, almost never been able to fully A rate ships all the time there. Between Lembava and Diaguandri, can usually A rate 80-90% of most ships, but vast majority of time, not been able to fully A rate my ships
always something missing - sometimes a major piece like shields or fsd, sometimes minor.
what i do is goto Lembava and Diaguandri for every ship outfit, get most parts A rated. Then have to hit up somewhere else for what I'm missing. Given the disparity of xp, I am suspecting station inventory is a) tied to specific ship and b) perhaps even some RNG based on player