Sounds Of Multi-Ply-Plated-Steel Quarters, Revisited

Revisiting this topic: http://goccf.com/t/175114

In that topic, I observed how MPPS quarters from 2001 to 2003 has a longer ring than quarters dated 2004 to 2013 when the ping test (balance coin on fingertip or knuckle, and lightly tap with another coin) was done. I did unplating experiments on some coins, but didn't find anything of value. Obviously, my sample size of 2, no control for time or voltage was used, and experimenter bias wasn't helping with things.

A few days ago I poured out quarters I had accumulated, and did the ping test for quarters (only caribou reverses were tested, as per the linked topic) dated 2014, 2015, and 2016. While 2014 makes the same sound as quarters dated 2004 to 2013, 2015 and 2016 were interesting.

Out of the eight 2015 quarters I have, three of them actually produced a long ringing, like the 2001-2003 quarters! All eleven of the 2016 quarters also produced a long ring. The ringing 2015 and 2016 produces a lower pitch, but it still is a long ring versus the 2004-2014 quarters which doesn't ring.

So for the question of "ringing" and "non-ringing" quarters, something must have changed. Is it possible that the RCM changed the composition or process of manufacturing their MPPS coins? Does anyone know the reason behind the difference or have a plausible explanation?