Excel formula calculation sometimes gives errors

[palmist]

I have a simple formula based excel (attached), Where columns :-
O will have value of column B, if it is the last occurrence of the value.
=IF(ISNA(VLOOKUP(B35,B36:D$5629,1,0)),B35,"")
P will have sum of column N, Where column B matches Column O.
=SUMIF($B$3:N35,O35,$N$3:N35)
Q will have sum of column M, Where column B matches Column O.
=SUMIF($B$3:N35,O35,$M$3:M35)


So As expected the rows 10 to 31 have values of P, Q as 0, wherever O is null, but why is row 35, 37, 43 and later showing up values in Column Q, when it should be 0 ? And even if it is showing up, why 4, 6, 8 ... why not the actual SUM ?

Please tell me the reason for this problem, since this excel issue is irritating me a lot. This issue just vanished when I put some value in H32:H39 , but why ?