I've read before (might have been here on TE) that the scales tend to be lower body biased (if you have more fat on your lower body, it will be higher than your "whole body" average) and the calipers upper body biased (if you have more fat on your upper body, it will be higher than your "whole body" average), while displacement is more generalized. Some of the scales (more $$) are apparently more accurate, all things considered.

I do mine occasionally, maybe once a month, while I generally weigh myself once a week (if I'm in an "I care" phase, anyway, I had a long "I don't care" phase ). It just happened to be a feature of the scale we bought, maybe a combination of calipers/scale would be better (if the scale is fairly accurate, anyway, which is still up for debate ).