Hmm. That confused me a bit, since that is exactly what REW tries to do if the room dimensions change - i.e. keep the head in the same relative position for length and width (but maintain the same absolute ear height). If you type new numbers into the length and width you'll generally find that is what happens. The problem occurs when using the arrows to alter the values, and was caused by using the same resolution for length and width as for head position - a 1cm change in length would mean a 0.38cm change in head position, but to the nearest cm the change was rounded down to 0cm so the proportion changed. I've fixed it for the next beta.