Two people in the same grade of job can be paid significantly different amounts of money for a variety of factors, for example the willingness of men vs women to negotiate initial salary.
Sorry but it sounds like your stating a very obvious thing that applies to man vs man, man vs female or female vs female. Everyone's personality differs so some are more willing to take a chance or not, not every person is willing to take their company for as much as possible. Some people feel that a companies superiors show know the staff they want to keep and keep them sweet as possible.
Sorry I want to share this since I dont have any friends that care, so here is a bit of a unload (Sorry OP, not trying to high jack. Its kind of related)
I moved into an infrastructure team at the start April this year and I haven't stopped working my ass off since. Some of it came from trying to impress, some from knowing the companies issues and genuinely wanting to sort them as much as possible and some from the fact I get to play with new stuff and learn. Another part of me is happy that someone once talk too was willing to see me as a possibility and give me 6 months to prove myself.
I'm going to be honest, I can be my worst enemy at times. I was extremely unhappy over the last few years and was open with it to some other colleagues, I made my mouth go about wanting to try my arm at contracting around desktop support/2nd line due the shambles that was my work at the time. But over the years I moved into my first home and had my priories change to wanting to be a free spirit to wanting stability. But that open unhappiness bite me in the arse when one of those colleagues (Contractor at the time) took a pay cut and came into the company and took management of the infrastructure team.
I'm very glade that he didn't hold my past attitude against me and he told me straight that he didn't want that attitude coming with me to the team for the 6 month secondment. I was clear back in the day why I had issues but I stowed it and parked it and worked my ass off. Funny thing is that he was due to tell me what was to happen at the end of September with the secondment but it came and went, I only just got a one to one with him on 22nd of December (For clarity I was meant to have one once a month every month, I had my second one in 9 months then). I have to say I was very nervous when I asked what was going on, he basically said that he want to see me sweet and see if I came off the rails at all (Two other staff started secondments in my secondment time, one just struggles and the other went off the rails cause he wasn't hand held and didn't want to wade through the piles of ****** jobs in infrastructure that no one wants to do (For clarity, work is work that's what your paid for! If you have free time do that **** until the boss has time adjust your priorities do it)).
So it all came down to him telling me that he will be keeping me on as a permanent infrastructure team member, just needs to do the internal processes to make it so after the new year, its a massive weight off my chest. Mind you it all boils down though to what your temperament is and how far your willing to go out of a limb with management in the end. I could have tried to push it more and sooner, but I just dug in on what needed done in the job and worked my arse off. I don't think there is always one way of skinning that cat, you just need to take the path that makes you happy and gets you where you want to go.
Next year I've got SCCM and management training booked and I'm now moving on to MBAM, SCCM and getting our other structure up for Win 10 LTSB setup ready to replace the WDS/MDT deployment setup for Win 7E I build from scratch. Fast, long, intense and brain draining this 9 months!!