Easy :
1) FC3 damage models are very simple compared to fully simulated planes. You should try some Mig-21 vs F-5 servers, you'd be surprise how many missiles these jets can take and still make it home, with a good bunch of failures.
2) F-15 and Su-27 are 2 big planes with way more fragile wings than the mirage, making them easier to damage.
Combine both and you get what's considered a kill way more often than it should be.
It's not only the mirage that behave like that but every fully simulated plane / helo ;)