I compared the outputs of mIRC and nslookup and they were the same here (unsorted). You can see the exact order of IPs that mIRC uses by using the script below and issuing eg. /dns irc.efnet.net

on 1:dns:{
  echo 3 I: $iaddress N: $naddress R: $raddress
  var %n = $dns(0)
  echo 3 n: %n for $dns(0).nick $dns(0).addr $dns(0).ip
  var %m = 1
  while ((%m <= %n) && $dns(%m)) {
    echo 4 %m dns: $dns(%m) nick: $dns(%m).nick addr: $dns(%m).addr ip: $dns(%m).ip
    inc %m

It may be that for some people Windows (or perhaps firewall software) caches the dns results and re-orders them in some way?