The only common one wh en i tcomes ot pay discrepancy is companies failing to promote employees quickly enough, or adjust after recessions or crashes.
I graduated in the middle of the dotcom crash. I was pretty happy to make 36k/year out of college (It wasn't in the US though, thus why the number's pretty low), when most of my friends ended up working at retail stores working the registers.
Then the effects of the bubble pop went away...and I was still getting silly 2-3% raises, even though the years of experience were piling up.
I switched job, got a 50% (!!!) salary increase. 1-2 year later (which would kick me in the next "bracket" as far as silly recruiting goes), still 2-3% dumb raises at the new place. Switch job, BAM! another 40%~ increase.
Fastfoward a few years, it seriously starts making a big difference. You can't give raises equivalent to inflation rate to people in a market that's changing so fast and where the perceived value of seniority is at a premium, else all you're doing is paying to train employees for your competitor.