10-day tourist visa

I went to China for Xmas and New Year's, spent 2 weeks there. Getting the tourist type was no problem. Requested a double entry oneon the day after I came back to Bangkok for Chinese New Year and end of March (my Chinese girlfriend's birthday) and I got a 10-day single entry one. Has anyone experienced this as well? do you know why this happened? I provided all the materials they requested, including both flight tickets and invitation letters.

10 lousy days!!! how is this possible?