For each level, there is a rate modifier that you multiply your AGI by to get your crit. For level 50, this multiplier is 0.02288. For other levels, this number is different.
When you are level 50, the formula is
%CRIT_RATE = CRIT_RATING*0.02288, where CRIT_RATING = AGI*4;
You use this same multiplier to calculate every other rate in your char window. e.g.
%PARRY_RATE = PARRY_RATING*0.02288, where PARRY_RATING = STR*2;
Chef foods can improve your crit rating, and dodge on gear can improve your parry, and so on. I dunno the modifiers for other levels, there was a guide with more detailed calculations somewhere around the forums but ... effort >_>
As you are lower level though, your crit rate is higher for the same amount of AGI, because the multiplier is higher for lower levels. The developer's goals here are to make your crit rate consistent with the AGI growth while levelling your chars, so that you have a similar crit rate throughout your levelling experience, at least that's my understanding of the logic