The Major League Baseball salary structure operates in a pretty straightforward, simple fashion, at least until you hit the evolving beast that is free agency. Young, inexperienced players make league minimum for about three years, and if they’ve been good enough to stick around to year four, they’ll be due a raise. The better they’ve been, the better the raise, and thus the recurring arbitration system takes place each year until they’re either booted to the side or reach free agency.
Two parts of that become pertinent: you have to be very good to get paid very well in those years, and teams that have those kinds of players can see their payroll vary wildly from year to year in the process.