Amazon and eBay have been great for getting thread adapters for just about anything. The possibility of getting a good thread adapter instead of buying (and waiting) from the country that uses that thread is a boon. But it also exposes you to all sorts of quirks.
Take 1/4" BSP, based on the British "Whitworth" thread standard. Some countries didn't like to standardise on a British standard, so you have German (European?) "DN", which is exactly the same thing for pipe threads but has a "metric" numbering system. The Japanese tried to consolidate all the "standards" to give "JIS", but again pipe thread standards are based on Whitworth. The Americans bless them had to go it alone and developed their own thread-form: There's some approximate crossover, like 1/2" BSP can be graunched into 1/2" NPT, but not 1/4" because the threads-per-inch is very different (you might get away with a short thread when one is plastic and easily deformed).
Then there are anomalies like a favourite of mine: 7/16" JIS fits very well on to 1/4" MFL (NPT) (the difference in size is 'cos the Japs standardised on thread diameter, the American NPT, and British BSP, on pipe internal bore). But JIS is based on BSP which isn't compatible (at 1/4") with NPT, so how does that work? Then there is the sealing cone, but slight differences in this probably doesn't matter if pressure is low (<4BAR?). And tapering: NPT is tapered, BSP may or may not be (BSPT and BSPP).
Head mashed yet? Mine is.
(EDIT: Change JIC to JIS. Small error but big difference!)