What’s the rhyme or reason on the lack of consistency with showing vaccine or other documentation traveling around Thailand?

As an example;

Traveling from Phuket airport to BKK i was not asked to show vaccine info. BKK to Phuket I was also not asked.

The ferry from Phuket to Phi Phi I was asked for it and told it is mandatory and again taking the ferry back to a Phuket I was told I must show it.

That doesn’t seem to make any sense.