The relevant standard for fire safety of seats was EN 45545-2: 2013, "Railway applications — Fire protection on railway vehicles" Part 2 being "Requirements for fire behaviour of materials and components". I found an
RSSB▸ report (T843, on Spark) about this standard dated January 2014, which says, about the materials standards:
Tests to the British and European standards were performed and the results compared. On the whole the results were comparable, in fact the results of the project gave UK▸ experts much more confidence in the European Norm.
However, it also had this observation about test methods for whole seats:
A difference between test methods of one classification level might be expected especially when it is considered that both test regimes have only three classes, however, the two test regimes produce totally contradictory results for almost all the seats tested. This is true to such a degree that a non-compliant seat to the British system (one that cannot be used on any train assessed using BS 6853) could be installed on an underground train (HL3, the most onerous requirement) if EN 45545 is used to test and classify the product.
Now, those might not correspond with what you expected. My guess is that, when the group called to work on the standard assembled, there were no experts on those test methods present. This is always a risk with standards - participation is voluntary and generally unpaid, for individuals and/or their employers, and it can be hard to persuade the people you really need to bother. This standard was prepared by CEN under an EC mandate, presumably issued via the ERA. In this field there is an older body, the UIC, that is in a sense a rival of the ERA and feels threatened by it - which doesn't help.
RSSB, and no doubt others around Europe, proposed changes as part of an amendment planned for 2015 (the initial version had some known incomplete areas). An amendment did appear, so the standard is now EN 45545-2:2013 +A1:2015, though I've not yet found out whether it addressed this issue.