Does over() in sp work with "fun=sum"?

Dear list,
I try to simply aggregate some gridded data, but it seems like there is no
difference between "fun=mean" or "fun=sum". Did i do some thing wrong?

pop.v = over(gridv, pop_u, fun=sum)$band1
summary(pop.v)

Min. 1st Qu.  Median    Mean 3rd Qu.    Max.
  74.00   89.00   97.00   92.89  100.00  105.00

pop.v = over(gridv, pop_u, fun=mean)$band1
summary(pop.v)

Min. 1st Qu.  Median    Mean 3rd Qu.    Max.
  74.00   89.00   97.00   92.89  100.00  105.00

Thanks,
Phuong