Why M4a and not mp4?

[Alastair]

I am advised that a file containing audio data only in AAC format can be named either .mp4 
or .m4a because it fits correctly into both the .mp4 and .m4a categories. Most servers, 
control points and music players understand this, but there are a few that insist on one of 
other of these filetype suffixes. In such cases it should be fine to simply rename the file 
from .m4a to .mp4 or vice versa.

For a while now music and radio downloads have been put into M4a files and I think this 
may be upsetting some of my renderers.  I say think because I am still trying to sort out the 
tagging but meanwhile why M4a which I understand to be an apple format rather than mp4 
which is the ISO standard?  Is this an artifact from the BBC end or from get_iplayer.
Alastair.