OK, let me try and illustrate what I have done so far
3 files
1948.xlsx (complate data)
1949.xlsx (work in progress, but enough to illustrate)
Overall.xlsx (contains the Pivot and data pulled from 1948/49 files

So for just one rider, my data on a sheet in 'Overall.xlsx' pulled from 1948 and 1949 is
Year/Rider/Meets/Rides/Points/Bonus/Average/1st/2nd/3rd/4th/Retired/Excluded
1948/Bob Smith/43/171/303/9/7.30/61/39/42/18/9/2
1949/Bob Smith/2/8/16/2/9.00/3/3/1/1/0/0

So on the 1948 Sheet, the average is gained from ((303+9)/171)*4) = 7.30
Likewise, so far for 1949 ((16+2)/8)*4) = 9.00

If those get put in a Pivot, the Average options would be Sum of Average which works out to 16.30 or Count Of Average which works out to 2.00 or Average of Average which is 8.15
It should be ((319+11)/179)*4 = 7.37

Any clearer?