@Nigel *************
because the practical effect is the same and the difference lies mainly in the terminology and the underlying agreements between Thailand and countries for whom the visa exemption applies. There is no pre-approval required and it's granted at the border upon entry (luckily without payment as many visa on arrivals require).
Many other surrounding countries use "visa on arrival" so it's natural for first-timers to Thailand to assume the terminology is the same. In any event, you know exactly what OP meant by it 🤣