I think it's based upon image quality in some cases, or if there's no cloud in one picture and cloud in another. But using the time slider can be a lot of fun, especially in the UK where you've got some overlays from 1945 and earlier in places (I believe Germany also has these, so you can see war damage in Berlin and that) and also, if you go to Guam and use the timer, you can spot the ever illusive B2 stealth bomber sitting out on a taxi-way.