Military aircraft doing very high rates of climb and descent for example are more than capable of confusing the system because it was not designed to cope with them. ... RA does not mean you are on a collision course at all, it merely means that another aircraft is going to encroach on a very large bubble with you at the centre.
TCAS II advisories are inhibited for very high vertical rates (> 10,000 fpm). So a military aircraft climbing in excess of this threshold will never generate TAs or RAs.
Otherwise, TCAS computes what is known as the Closest Point of Approach (CPA). Version 7.0 introduced a filter called "Horizontal Miss Distance" (HMD). The HMD differs with altitude, but basically tracks another variable called DMOD which varies from 0.2nm at 2000 ft to 1.1nm at FL200 and above. Note that these values are considerably smaller than ATC separation minima.
TCAS also computes the time to the CPA, called tau. Above 10,000ft, the tau threshold for an RA is around 30 seconds.
So if you're in cruise level at FL360 and there's an intruding aircraft (military or otherwise) climbing from below at brisk rate (say at 8000 fpm), then an RA will be issued if the intruder is predicted to cross inside 1.1nm of you horizontally within about 30 seconds. That could be a 1nm "near miss", or a direct hit collision -- there isn't enough precision to know.
What TCAS can't measure is intention. Maybe that intruder, currently at FL280, will level off at FL320. In that case the RA would have been unnecessary. But perhaps it was cleared to FL400? You just don't know. That's why you
always follow the RA.