The springs on a new mag can be really stiff for some people to get the last 1 to 2 rounds in. They say to load it up the mag with out the last round in it and to leave it that way for about a weak to loose the spring up to get that last round in. When I personally get a new mag (regardless of capacity) I all ways load it up to the last round an leave it that way for a couple of weeks . Loading the last round for a couple times might hurt the thumb but I push threw that lol

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The fun way to loosen a mag spring up is to get out a put quit a few hundred rounds threw the mags. Cause the constant loading and unloading is what weakens the spring to make loading it up to full capacity a lot easier.
As far as the number on the mags go, I have no clue why it is a 33 rounds but only has 31 on the back. My guess is that it is an after market mag that was made cheaply but then again I truly don't know.
This little product works wonders with getting that last couple stiff rounds in:
http://www.glockforum.com/forum/f11/easiest-way-load-glock-magazines-3560/