Let's nail this one once and for all. mjd - sorry, but Morwen is right.

Ignore the fact that one is a word beginning with "t" and ending with "k", and another begins with "p" and ends with "y". You can call them "elephant" and "K-12" if you like. Really doesn't matter.

OpenStreetMap is an international website. We have to make our tags relevant across national boundaries.

So in the UK, the green-signed A roads are tagged in OSM as "highway=trunk", and the black/white-signed A roads are tagged as "highway=primary". You cannot blindly assume that there is a particular equivalence to any single UK scheme which is your "accepted version of life", especially as that's probably received opinion rather than anything official. You see, there is actually no current UK scheme which uses "primary" and "trunk" in conjunction - just now-outdated (but still copyrighted!) OS maps. Seriously:

England/Wales government (statutory)	England/Wales government (public-facing usage)	OpenStreetMap	French government
special road	motorway	motorway	autoroute
trunk	strategic	trunk	voie express
class 1 road (primary route network)	primary A road	trunk	(no such distinction)
other class 1 road	non-primary A road	primary	route nationale (N)
class 2 road	B road	secondary	route departmentale (D)

(It's different still in Scotland, where all green-signed primary roads are, since devolution, "statutory trunk". That's certainly not the case in England & Wales. Go figure.)

And the postscript which is actually more important than the rest of it: OSM is a social mapping project. Convention is defined by what other people do. The majority will of regular OSM map editors is that green-signed A roads are tagged "highway=trunk", black/white-signed is "highway=primary". It may seem confusing to you right now, but believe me, there are plenty more confusing things to discover about OSM. :)