Sounds OK to me. All Ryanair is doing is spreading the cost of maintenance over the period of time that maintenance gives a return to the company.
Also, while this periods maintenance costs may not all hit the P&L today, costs of maintenance performed over previous periods will.
The net effect is a smoother (and therefore arguably more realistic) report of finances.
Of course, none of this matters if you really, really hate Ryanair anyway
And they say love is blind
Regards,
V.