I very much suspect that Ryanair's definition of passengers fits in with general accounting principles. I'm sure there are others out there who know far more on accounting on me - if I've got it wrong, please say so !
As most know, Ryanair generally do not give refunds unless they absolutely have to. Thus, if you pay for a flight but do not show up, Ryanair gets to keep your money. In accounting terms, this means the funds paid by the customer can be "recognised" as revenue.
If on the other hand, Ryanair cancels a flight, the airline is obliged to refund the money paid back to the customer. These funds cannot be recognised as revenue.
The volcano in the spring was extraordinary, and it is likely that many airlines will show their monthly stats to include both recognised revenue and also separately "customers who booked but had to be refunded because of the airspace closure".
Investors and auditors may well accept the volcano as being an extraordinary event, and will take the separate "what if there had been no eruption" figures when examining the company's overall health. Air traffic control strikes are infrequent but not unusual and would not be deemed extraordinary. Thus a company should not normally expect to be taken seriously if it manipulates the stats for ATC strikes.