Let's say we are talking about the statistics from the Pilgrim office in Santiago (read: people claiming a Compostela). Let's say, you check in at June 5th, in the form you state you started in Leon, are French national, travelled by foot and for religious reasons. So in the statistic it will reflect one pilgrim starting from Leon, on foot, religous reason, French national, arrived on June 5th. It will not reflect when you started, if you walked in one go or where you are currently living.
As with all statistics they are not perfect. They give some indication if you compare them over the years. But they won't be able to count those who did not claim a Compostela, they can not count those that abandoned the Camino for whatever reason and they can not tell if every statement is made truthfully ( i would assume that many claim "religous reasons" just to make sure they get the Compostela).
It gets more complicated when you compare statistics from different origins, for example the one from Saint Jeans pilgrim office with Santiagos pilgrim office...