Simon is correct they only count those who preach; specifically, they count them in the month previous to when they receive the report. The 35% who are irregular will most likely be jostled by the Congregation Secretary (or nowadays their Book Study Conductor, or whatever they are now called) until they submit a slip with a phantom hour or so for whatever months are missed. These hours - and more importantly, publishers - are counted for the monthly report that goes in after they arrive.
Now the reason this is important is that it makes a publisher appear twice or more in a given month. That is, the April peak may include Bro. Smith's time from February, March, and April, and count him as THREE PUBLISHERS! This was admitted a couple years ago during the CO visit at my old KH, I think during a regular meeting but possibly during a meeting with the Ministerial Servants and Elders. And if you know that the CO wants to see an April peak, when do you think the jostling will be the greatest?
So the Watchtower OWES its peaks to irregular publishers. But, because people are catching on to them, they are finally pushing publishers to report monthly, so that the peak months and the other months (along with the average) are not so vastly out of whack. Despite this, in recent years they have given more attention to these known faulty peaks even when everyone is starting to realize that only the averages even approach validity.
Bodhisattva