(=AVERAGE(F22:INDEX($B2:F22,$B$25,1)) Don't understand formula?

C

Claire G

ScrapA ScrapB ScrapC ScrapD ScrapE Scrap F
Dec 27.53 27.61 27.43 28.63 28.83
Jan 27.55 27.77 26.65 28.24 29.18
Feb 28.35 28.6 28.44 28.36 29.42
Mar 27.14 27.74 28.02 27.1 27.54
Apr 28.67 28.04 28.09 26.79 27.25
May 27.86 28.41 28.3 27.9 27.74
Jun 28.51 28.56 27.96 28.04 28.32
Jul 28.16 27.84 27.76 27.17 29.64
Aug 27.59 27.69 26.6 27.75 28.45
Sep 28 27.5 27.38 27.4 26.9
Oct 27.94 27.99 27.06 28.33 27.65
Nov 27.89 28.2 27.69 28.4 29.54
Dec 28.66 28.76 28.64 28.36 28.31
Jan 27.94 27.81 27.69 28.04 27.73
Feb 28.13 27.65 27.31 26.27 29.78
Mar 27.03 27.68 27.88 27.1 27.38
Apr 28.17 28.23 28.12 27.96 28.5
May 27.21 27 26.83 27.31 26.47
Jun 27.8 28 28.2 27.88 28.07
Jul 28.12 28.4 28.35 28.75 29.32
Aug 27.91 28.39 28.11 28.18 28.16

12 Auto Average 27.99 (=AVERAGE(F22:INDEX($B2:F22,$A$25,1))


I am attempting to use the above formula for a rolling average of
10months(Nov-Aug) from Columns B-F, it works but I am not sure how to
recalculate when Sept. comes around (becasue then I will want
Dec-Sept)? Am I using the wrong formula? I realize that next month
after the Sept figures are entered, if I tweek the #12 to #13, it will
recalculate but I don't understand the formula ...can someone explain
what this formula does and if I have chosen the correct one to use for
this function?
 
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