A better question is, why is there a need for *every* trade code to map to specific background skills?
Some planets have no trade codes at all (eg, C553777-7). Should they generate specific background skills?
MegaTraveller introduced the concept of background skills based on homeworld UWP, but didn't use the trade codes to determine this as such. TNE expanded on this. Mongoose have chosen to simplify things by instead using the trade codes as a basis for background skills, but as a result there are a few oddities, such as homeworlds with trace atmospheres only giving Vacc Suit 0 when there is hydrographic of 1+. Simply put, Vacc Suit 0 should be a background skill for any homeworld of atmosphere 0-1 and 11-12 (i.e. those atmospheres that *require* environment suits).
By my reckoning, it is also questionable to give Seafarer 0 as a background skill to Fluid Ocean worlds, since on average they have less hydrographic coverage than similar sized worlds with water oceans. It seems silly that if you come from a world with 10% ammonia based seas that you'd automatically know how to operate a boat, while someone from a planet covered by 90% water won't. I'd make it than any homeworld of hydrographics 8+ should give Seafarer 0.
There is also an issue to me that Flyer (i.e grav vehicle operation) is absent from the background skills list. This should be based on tech level, probably from TL 9 or 10, but definitely from TL11.
You can also question how much contact with animals a character from some Poor worlds will have. Poor does not equal low tech; many are non-agricultural and none are agricultural. Someone from a planet with the UWP A120977-B knows how to ride a horse, but not how to pilot an air/raft? Suuuuure...
EDIT: Couldn't get this out on my head, so I've knocked up some rules. Posted on this thread:
http://www.mongoosepublishing.com/phpBB2/viewtopic.php?t=44641