Why do the video's on youtube reload or re-buffer when i switch to full screen?

Off and on for almost a year now ive noticed that sometimes youtube video's reload or re-buffer when i switch to full screen view. I click to watch a video with the regular small screen, it starts to load and play the video, then as soon as i switch to full screen it starts over from the begining, It doesnt happen all the time though. Why does this happen? How can i fix this? Does this only happen to me? Serious answers only please