Your Pokemon is working its way towards it's level 100 stats, so the stats are gradually added each level until it gets there, the problem is if you averaged out the amount of stats you'd get over the levels it has left to go, you wouldn't get a whole number, in your case, the number is somewhere between 2 and 3 for it's attack. Since it would be messy for it gain 2.35 every level instead sometimes you it 2 and others 3.