Keep archiving same stuff, different sizes each time...

[imzjustplayin]

I've been archiving the same files multiple times with Winrar and the same settings but every time I do it, I get a different file size. I'm very confused, why are these group of files getting different file sizes every time I try to archive the files. I get sizes in the range from 11.7Mb to 16Mb, I've tried solid and non solid archive but the results are inconsistent. Is there any reason for why this would happen? I have a Pentium M which varies the processing speed and I've found that these files are archived at a surprising speed considering that it's 29MB worth of PDFs, maybe it has something to do with that where the program incorrectly detects the computing speed.