I was under the impression that each player was  allowed only 5 settlements. I've never been able to build more than this number (although I have tried multiple times), yet I've played in several games where either one or more of my opponents has had six on the board (I remember specifically one player named CommonBrick, or something to that effect, achieving this in two matches). Just wondering if I'm missing something in the rules here, or if this is some kind of glitch or exploit?

I've also started the game with only one settlement (because it took awhile for  people to join the match and I was cooking spaghetti and was unaware that the game had started,) apparently, somehow, the auto placement put both pieces on the same tile, clearly a glitch here... or in the very least a penalty for not paying attention. Oddly enough I went on to win that match.

Regardless, does anyone know the specific rules regarding settlement placement? Everywhere I've looked claims that five is the limit.

The only way to build additional settlement after the initial 5 is to convert one of the settlements into a city.  So, if you converted 4 of the 5 settlements into cities, you could actually build 4 more settlements. 

Thanks for that info. This was not the case in any of (at least the most recent matches) that I've found this in. In the last match neither opponent had more than one city. In fact I had three cities, the longest road and was well in the lead with 8 points. Do you know where the "official" rules can be found, I can't seem to find them anywhere. If this truly is a "glitch" though, it seems that it's one that has yet to work to my advantage. Anyone else experienced this?

Oh... I misundertood you. That clears it up, somewhat. Regardless, though, I've had five settlements and multiple cities and I've never been able to place a sixth settlement. Are there a limited number of settlement pieces per board regardless of player?

Yes. There are a limited number of game pieces. 5 settlements, 4 cities and 15 road segments.

You can see how many you have left by pressing the left bumper.

If you converted 4 out of 5 settlements into cities you could only build 1 more settlement. That would give you 10 VPs.

Yep