Odd, I've often seen a retrained unit drop from 1 to 0, or from 2 to 1 chevron.

As was stated, the chevrons displayed is the average for all the men in the unit. In past TWs, the game did keep track of every individual man, and bonus' were applied based upon that individual experience. In RTW and M2TW a possible bug was present that new troops entered with an experience equal to the units average experince thus making it easier to improve the quality of your troops. In STW and MTW it worked where replacement troops were always green (exp 0).

It is possible that the variance that others and I am seeing is completely due to the rounding that occurs as the game averages a units experience for display. I admit that while I often see a drop of 1 experience, I've never seen a unit drop 2 chevrons, even if it was down to only a handful of men. Would love to hear CA drop a simple/quick explanation of how exp and recruiting are done. Of course, I'm sure folks around here will have it figured out fairly soon anyhow.