I can certainly confirm that the iService app offers the option to extend the validity in 2 ways. Either "buy validity", THB 8.41 for 90 days for instance, or by topping up credit, Different amounts give different extension periods (I did mistakenly mention THB 50 for 90 days, actually it only in fact extends mine by 30). But whatever method is used, the validity can never exceed 90 days. I tested it a long time ago, bought 180 days for THB 14, result was extended from whatever it was, maybe 10 days, to 90, not 180. Queried it with True and they confirmed that my SIM (30 day Infinite Thailand) cannot ever have over 90 days validity.
It is quite possible that not all tourist SIMS are created equal.