@Justin *******
The only stupid theory in here is that at a certain moment visa requirements stop. Nobody can exactly tell when and why that is logical.
5 years of dental treatment is not logical, but why should they make it overly complicated with hundreds of different valid dates.
One treatment might take 7 months, another 8 months, a third 12 months, not to speak from treatments where the end date is not known at the beginning.
Therefore it is perfectly logical to just have one validity termin, but the visa is only valid for the specified purpose of stay.
Very easy, but also unconvenient for the people that are not really using the visa for the purpose, but just book a course or seminar or dental appointment to become a visa to live in Thailand.