Well using the MrMalty Pitching Rate Calculator it says that you should have used a 1.58 Liter starter, or 1.67 Quarts. I just plugged in the OG of 1.052, 5 Gallon batch, and Simple Starter. Give it a try:
Mrmalty.com
I'm not sure if that is why it didnt attenuate as much as you wanted, but could be one of the reasons why.