I've noticed a trick when watching long HD youtube videos that I need to pause and re-wind several times. If I'm signed in to my web browser the video quality i

I've noticed a trick when watching long HD youtube videos that I need to pause and re-wind several times. If I'm signed in to my web browser the video quality is demoted to lower quality after a while of watching, pausing and re-winding. To avoid this degradation of quality, I use a browser in incognito mode, and the quality stays HD. Does anyone who why this occurs?