The positional problems are likely frequency cancellation. When you have multipath signals coming in like Steve said there are areas in the room that those signal paths cross over each other. And depending on how the frequencies match up with each other they either cancel or reinforce the signal strength. Also know as an interference pattern.
This is usually in weaker signal areas but can still happen in stronger areas. In my driveway, my normal parking spot I have a repeater that comes in weak and intermittent. If I back the truck up 4 feet the repeater comes in perfect. When I was transmitting to the repeater it made no difference which spot I was in, came in fine.
When I'm in my house on my HT inches make a huge difference. And the funny thing is for the most part it doesn't change. We'll have to see as the leaves drop off the trees.