It is the BEST 10 scores of the "most recent 14". In other words the 4 HIGHEST scores from your "most recent 14" are DROPPED from your average.
Having said that - if you have a REALLY GOOD ROUND (say a 32 for 9 OR a 66 for 18) and it is the oldest one INCLUDED IN YOUR AVERAGE - then what happens is that older "really great score" (that you may have forgotten about since you have shot 13 rounds since) now DROPS from your average and may be replaced by the newer "really great" but HIGHER score.
I see this happen frequently in my own average where it will suddenly jump UP - and it is because a GREAT round I had a week or more ago now no longer counts.
The bottom line is your scores need to CONSISTENTLY be around 65 (or lower) in order for your average to "catch up" and actually reflect that number.
One more comment - I also recently turned "Master". Expect that for the foreseeable future, it is very common for your average to actually go UP - not continue down. You are pushed back to longer tees which of course results in adjusting to the course from the new distances and all of its inherent "challenges" - landing in sandtraps you never landed in before, trees that were never a factor before are suddenly a factor in your Drive (see BPB # 13) and so on - so don't be surprised if your numbers spike UP a bit until you get a handle on how your game now plays from the longer tees, and then they start to come down again. I AM IN THAT SAME BOAT WITH YOU AT THE MOMENT. :-)
Hope that helped, AtlantaCoaster