What Causes Low WBC Count In Blood?

Hi I have recently changed to being a pescotarian for over a year and rarely drink. Play football, workout, work full time and feel healthier than ever. I haven t been ill in a couple years and have recently gotten a blood sample back for Insurance purposes with what they called a decrease in white blood count??? Is this normal for my diet? I also had an hiv test that was negative and all other readings were perfect? I m 5.9 10.2 in stone and don t smoke either.