I also notice that sometimes the result changes slightly on same data, I have applied the one you sent to my same sheet and I get the same answer but then if I click with in the formula cell it will change the answer from 309 - 249. I can see no difference in the formula's when I match them mine is the same is yours.
=AVERAGE(IF(((($D$2:$D$69)*1>=W$2)+(($D$2:$D$69)*1 <=W$3))*($C$2:$C$69=X2),$K$2:$K$69))

Why would it do that? I should get the same answer - all the data is the same