Yah it could mean that. Or it could mean that you are making some mistakes in big pots, which I would imagine would skew your river call efficiency more than mistakes in smaller pots (assuming they were happening at the same frequency).
I've tried googling good explanations but havent found any that are really good or conclusive. If anyone finds any let us know.
You can also try filtering for spots where you faced a bet on the river, and what your won at SD % is. Because WTSD / Won at SD also includes a lot of spots where river goes check check, or hands are checked down etc.