CD/DVD Drive speeds are to be taken with a pinch of salt. Please read
this FAQ, and also take a look at some of the software on that webpage.
Basically, if a drive said it was 100x, it may mean that it can only write the last few MB of the disc at that rate. Also, there is a possibility that your drive doesn't like your media very much and is clocking down its speed. There is a comprehensive media review/information website out there but it escapes me at the moment.
Also, check out some of the reviews of similar drives at
CDRLabs and see whether you get similar results as they do.