nmboxer asked a great question: Why DO players wear different jersey numbers anyway?
There are a couple reasons that I can think of, but I welcome anyone to share their thoughts on the subject!
For players that played around the time uniform numbers were first introduced, the POSITION of the player determined their uniform number. So, if a player was moved around the field, they may have worn a different number just because of that.
Some players (like Thome, for example) have an affinity for a particular number (his is 25), but when they join a team, someone else may already have the number they want and for whatever reason, the other player will not swap numbers. Once that number comes open, then a player may switch.
In a similar vein, if a player wears one number then leaves, another player may get his number. If he returns to the team, that number might not be available any longer, so he must choose a new one.
If someone can think of other reasons, please feel free to comment.