Note this isn't true in all countries. My UK cards within the UK all follow some apparently online process in any UK merchant, however during a stint in Finland a few years back, I didn't find a single example of a merchant where their reader didn't instantly approve my transaction as soon as I correctly entered my pin.
Never received a (note: I know, we can all make guesses) conclusive answer explaining the difference.
There's no technical reason for the difference, simply it's up to the merchant & issuer that can arbitrarily set all kinds of limits where certain checks will be required.
It's a tradeoff between convenience and security, different places will make different choices, but generally competitors within a single market (e.g. UK or Finland) will try to make the same choices so they don't get customers asking why they're slower than a competitor.
Never received a (note: I know, we can all make guesses) conclusive answer explaining the difference.