I have no idea if this applies to the windows versions of Poser.

On a mac, your render cache for both PoserPro and P8 are stored in the same place. If you bounce back and forth between apps to use the render in background feature of pro, and or you have different settings of how many renders to save in cache between P8 and PoserPro....

Couldn't under stand why when I tried to compare two P8 renders I got  a test render I had run on PoserPro instead of the two P8 renders...and the fact that my P8 render cache had been erased because I have the render cache in PoserPro set extremely low at 2 and in P8 I have it set at 4. 

Filed a bug report and hopefully they will change the path of the cache to align with the version number like they do with the settings in SR2.

Gary

"Those who lose themselves in a passion lose less than those who lose their passion"