Why does my federal income tax change each paycheck? i think i filed it right?

each statement i have has made it change a point or two in each direction based on how much money was in it. if it was more i payed more if it was less i payed less is this the way its supposed to be? it usually stays about 5% if that helps.
Update: how is it determnined
how can i know how much will be taken?
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