Why does announcing "Runway Two Zero" allow the last digit to be cut off
while announcing "Runway "Zero Two" prevents the last digit from being cut
off?

It doesn't. It prevents the last digit from being cut off =unawares=.
It acts as a checksum of sorts. If all runways are two digits, then any
transmission missing a digit is suspect. If some are one and some are
two, then there are some cases where an error would not be noted; this
is especially the case with runway 2-20, both of which exist on most
airports for which one exists.

Jose
--
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