I noticed on blood work done a few years ago, lymphocytes were total 2.9 at 33% total and 5 months later total 1.6 and 21%. what could cause that drop? or normal variation? blood test done a month later lymphocytes were 1.9.

That's fine: They fluctuate wildly even from day to day, with the minor stresses and perhaps microbial exposures of everyday life. Ignore the percentages entirely; the totals are both fine. Keep in mind that they're your friends.

Probably normal: Different blood cell types will fluctuate within a "normal range," but common reasons that blood counts and cell types will fluctuate include all types of physical stress, like injury and inflammation, bacterial and viral infections, including influenza and the common cold. There may also be fluctuations as your immune system responds to environmental factors of which you might not even be aware.