Yeah that can be frustrating, and usually it reminds me of the real reason I'm folding. I get the same thing sometimes with the SMP client, they will assign me a1 WUs instead of a3. My i7 860 does about 8000 PPD with the a3s and a little over 2000 PPD with the a1s but sometimes they assign the a1s because they are temporarily out of a3s or they need to get more a1s done. So perhaps there is a similar issue in your situation. I'm not sure if you're running the uniprocessor client or the GPU client so I didn't check.
A good place to see what's going on is www.foldingforum.org
if a new issue arises you will usually find threads about other people having the same.