I have absolutely no scientific basis for this at all, but I do wonder how much of it is how your immune system and body 'reacts' to the sensor. I can always tell when I'm not going to get that long out of it, because it tends to be sore and itchy from the minute it goes in, whereas the ones that last and last, I can barely tell it is there. Sometimes, if it only lasts a week, and it's been sore and itchy, the site looks....awful.... when I take it out.