You'll be less confused if you realize that it's not an M16. As such, the Bushmaster XM15E2S series of rifles are named whatever Bushmaster calls them.
As explained before, civvie uppers are called "A3" or "A4" interchangeably with "flat top". This sort-of reflects the military designations, but not really.
It's kinda like how everyone calls every AR-pattern rifle an "AR-15" or every AR with a 14.5-16" barrel an "M4". It's technically incorrect in almost all cases, but it's the common names of the weapons as far as the general public is concerned.