It's backlash. Your mount can only correct for X amount of it based on your mesh. If you're slewing and seeing the same result, that's the best you can do without remeshing.
If you think about it, your heavy RC is causing the gears to mesh 'better' and is removing the backlash so it can properly center. Your light ED80 can't take out the slop enough and it's giving you what it can.
You can test this by adding a 10-20lb weight to your ED80 setup and doing a flip. Or, do something like the G11 guys like to do with a string and a weight around your RA Axis.