For Army it is based on a national order of merit list
top 10% get their 1st qualified choice
top 20% almost all get their first choice
between 20-50% its a bit of a crap shoot
below about 67% you are off to National Guard / Reserves.
In the end - 65% got their first choice
87% got one of their top 3 choices.
(actually more complex than that, need to read)