WE SHIP INTERNATIONALLY 
When trials biking was first starting they stipulated that the rear tyre must be 20inch in diameter. The bike riders wanted tyres with more cushioning for their rear wheel so Monty took a 1inch smaller old scooter rim and tyre (which is nominally 20inch in diameter). They then used that tyre and rim with an ISO of 387 instead of the standard 20inch tyre with an ISO of 406. To conform with the bike rules they printed on the tyre 20"x2.5", unfortunately this causes more problems.
To help differentiate between the two tyre sizes the non-standard small rim ones are called 19", some of the more modern tyres also have this printed on the side.