I think its because of the spinning magnets. I brought my mom a pair and plan to buy my wife one in the near future. (I made the mistake and went up 3 sizes. The first pair I bought for my granddaughter was her size but they were too short. 5 Ordered 6. I am not the first one to say that. At first, I thought they were going to be too small/short for my feet, but after I wore them for a day, the cork adjusted to my foot and theyre as comfy as slippers. --7) Call Quality - Untested. LOL! I havent had a day yet where these ran out of battery power during an 8 hr.