Wht would an employer take out federal taxes on some paychecks and not others?

I worked for the same employer this entire year as well as last. Federal taxes taken out last year on 45000 annual was about 950. Federal taxes taken out this year on 47000 annual was 260 for the whole year. Exemptions were not changed between the 2 years. this year, they took federal taxes out on some checks and took no federal taxes out on other checks, even when the gross pay for those checks were the same. Why would they do this?