Penko, I think it's just laziness and you hear it all the time from both pilots and controllers.
wun
too
tree
foah
fife
niner
All of which get routinely pronounced in the "everyday" way for the speaker, which is fine if the listener is another native speaker from the same part of the world/country, but not otherwise.
The other numbers seem relatively immune thankfully, it's just those 6.
G