Microsoft Access-queries

I have repeatedly had this problem
It either occurs when simply I try construct a query in access using data in a table or try compare to sets of data using two queries I have already constructed
The issues is that the numerical values in the data sometimes double, or increase in size!
I have sort of figured out how to solve it...
when I am contructing a query, I put the total for the numerical field as as "group", instead of "sum"
And when I am comparing two sets of data, I keep the one numerical field as "group" (total) and the other as "sum" (total)

There is probably a simple reason for this, but it is completely confusing me!

DO you know why it does this???