Originally the 900 prefix was a shared default code. The prefixes are assigned by ICAR, some are used as country codes, others are manufacturers codes, Trovan use 956.
A 956 microchip anywhere in the world means that it is traceable by Trovan to the country and the local supplier, the local supplier then tracks the chip by recording which Implanter received it, Impanters should be keeping records of which animal they put it in. But an end-user database like petlog is there to register the owner details associated with a microchip.
For 900 chips, traceability is problematic. There are many companies in China producihg 900-chips, some record where they went, others don't. They rely on the final supplier of the chip to record the details.
If, as is the case quite often, owners don't register on an end-user database, there is no system for backtracking. Even if they do register, the obligation is on the database to refer someone checking a chip on to where a chip is registered. Not all the databases cross-search. In the US the advice is to register on at least 3 databases if you have a 900-chip. Considering the failure of many owners to keep one database up to date, this solution is unlikely to work in most cases!
When a pet implanted with a 900 prefix microchip is brought in, it is a problem. Rescue staff, animal control/welfare officers and veterinarians and vet staff are busy people. They don’t have time to wade through the plethora of microchip lookup tools and websites. They don’t have time to email each manufacturer or sit on hold waiting for a customer service representative that may or may not be able to help them. They may have to call all six companies before they get the right one and they may not even realize these companies exist!.