Don't know why your meter hasn't reset... unless it's just a coincidence that it happens to be displaying the same as what your lifetime average is. I don't believe there is a fifth digit for the miles on the trip meters, so once you hit 10,000... it should be back to 0000.

As long as you're using the green hybrid site to record your mileage (are you???), there's no real reason to tie up trip "b" for lifetime averages.

Since I hit 10,000, I've been using my "b" meter to track short segments and daily rides (trip "a" is still used for tank measurements). Very helpful in finding what helps and hurts MPG on my route!