RE: How much do antlers change from year to year?
The amount of change is related to genetics, stress (winter/food/water, etc), nutrition and maturity. All things being equal it is possible for deer( be it WT or Mule) to put on several inches of antler growth from the previous year. Just as it is possible for either to lose inches due tomaturity levels (over peak), low nutrition and heavy stress.
To your question is it possible for your mulie to grow a 5" kicker in one year...absolutely. Wether its the same deer? Only you can answer that as you have all the evidence to cross reference and come to that conclusion.