Your formulas in C2:I2 find the last numeric value in row 4 and then sum the column according.....but in July you have some extra columns, so as ChemistB says you get the numbers from column AB because that has the last numeric value in row 4.

If you want to always sum the column with the header "Monthly Average (Excl Saturday)" then try this formula in C2 copied across

=MATCH("Monthly Average (Excl Saturday)",INDIRECT(C1&"!$3:$3"),0)

alter E1 to remove the trailing space