I wouldn't even feel comfortable speculating in the event that there's a medical condition at work here. Sit down with your doctor and block out 30- 45 minutes to go through your tests and then come back to bikeforums and tell us what the results are and then we can look at your training.
Often normal people can fall out of the "normal" range. There is also a genetic component to this- people with African heritage often run a lower white count than the population in general. The trick is to see if you can dig up old lab results from years ago. It's reassuring if the white count has always been low.
Otherwise, it could be due to a viral infection in the past few months, so I'd recheck it before getting too worried about it. In rare instances, it can be due to serious illness, but if you've been feeling fine and not getting infections, then it's likely ok; you can just follow up regularly to make sure it's not heading down.
A quick internet search reveals that overexertion can cause an elevated white blood count. I saw nothing to indicate that exercise causes low white blood cell count. Since humans are built for exertion, it would seem reasonable that using our muscles would make us more, not less, resistant to infection.
However, I am not a doctor. Since you state that you have been 'retested', it would appear that you are already consulting a doctor for this. That's the best course of action.
Have another blood work done. I have a physical at work every year since I'm on the Hazmant cleanup team. Every year the platelet count has been very low. The doc looked at the first one and said the test should've been done in citric buffer, not phosphate. After three years of hemophiliac levels, my wife scheduled me with a very good internist/oncologist who had the lab test my blood in citric and heparin and also do a manual count. Numbers came up normal(as I expected).
The tiny needles used on some blood collection vials can lyse the cells, phosphate buffer can cause clumping/falling out of solution.
Low white blood cell count could indicate a problem with your immune system caused by any number of things. But of course your doctor would know all this so I'd talk to him/her about it.