Oh so you were just looking at the voyage fare without the taxes? I was looking at the entire price.
I first looked at the base price of the cruise. After you said the PC cruise cost more than the TA, I checked again. The PC's taxes are higher than for the TA, but the total price for the same category cabin was still lower for the PC cruise for the least expensive cabin. I can't check DCL's site right now because I am getting en error message. (It must be overloaded!)