Premiere Pro inconsistent export times


Jan 24, 2012
I'm using Premiere Pro to edit videos, and it works great. (to some extend)
I tried editing and exporting a 12 minute long video, takes about 12 minutes to export, which is great. I also tried a much more editing heavy 1.5 minute video, also renders in under 2 minutes.
Then I tried another video, 7 minutes long, there is almost no editing done for this one, only some music and some audio being lowered at certain places. This video takes 25 minutes + to export. Why is that? I can still do the 12 minute video at the same speed, so I'm just wondering why this 7 minute video takes so much longer to export?

PC Specs:
Phenom 2 X4 965 (OC'ed to 3.7GHz)
GTX 560 Ti
2*4GB Ram
100GB+ space on HDD I render to