I once had a patient on Vancouver Island, and on a routine annual check, we found low neutrophils. This kitty was otherwise ok, and in great shape. No other issues cropping up on exam or within the bloodwork! What we decided to do was test this little guys white blood cell count every couple of months over the course of a year. And what we found was that his body went through a natural cycle - slightly low, then higher, then slightly low.
Since then, I've seen many cats who are healthy, yet reveal slightly low white blood cells on their routine wellness scans. Some of these guys seem to have a natural cycle, and some cats just never read the book - they seem to have a slightly lower count and actually it's normal for them. Some 10% of patients will lie outside of the reference range as their individual “normal”.
What I suggest to folks is that if they want to know for sure, then recheck the CBC in a months time.
An overwhelming infection that will cause the neutrophils to drop almost always equals a cat that is very obviously sick (fever, lethargic, septic), along with other telltale signs on the physical exam or bloodwork. There is no doubt in my mind that you will know this to be true if it is happening.
P.S. - there is good chance that our next cat will be from Cuba :)