My platelet count was 143 the lab says in normal range but last month it was 166 what could possibly cause this drop and i thought 150 is normal?

Different labs, different ranges. Your platelets are at the lower limit of normal and there is not enough for a trend unless they can be correlated to a known or observed condition. Further tests will clarify if a clear trend and your ordering doc will remain your best reference in putting them in real context (missing here as yo why you have them checked).