hmm, quite the kunundrum indeed. To be honest, 10 isnt really high, but I can understand wanting to lower it. Which test kit are you using? there are some that have a reputation for being inaccurate. Aside from that, there must be SOME sourse of nitrate... whats your feeding regimine like?
Also, is it homemade RO or from your LFS and have you tested it before it goes into your tank, some LFS RO/ready mixed is not up to scratch. And make sure you clean out your mixing buckets often if you do mix it yourself because contaminants will build up over a short period. After your next W/C halve your feeding and test again after a week. Good luck
the live rock rubble has only gone in 5 days ago as i wanted to get rid of the ceramic noodles cos i was told they could be a nitrate factory and yes the RO is from my lfs and i have tested it and it is perfect but i havent washed out my mixing bucket 4 a bit so that could be a problem