Perhaps I should be posting this in the framebuilders subforum, but I opted for greater traffic.

I've run into a snag with the Ted Williams I'm working on. I checked the alignment using Sheldon's string method, and after minor adjustment the chainstays are aligned. However, when I put the rear wheel in and center the rim between the chainstays, it's off-center vis-a-vis the seatstays. I thought maybe the wheel wasn't dished properly, but if I mount the wheel backwards, I get the same measurement. It's about 21mm from the rim to the seatstay on the non-drive side; 26mm on the drive side.

I haven't yet done anything with the dropouts (I don't own the tool) but they appear to be reasonably straight. Is it possible for misaligned dropouts to throw the wheel off in that way? Or do I just have a funky frame? Any alternative theories, or ways to check various alignment issues without a frame table?