Yeah, those line differences were part of different ORIGINAL mixes, not specific to DVD releases (tractor beam line being from the mono mix, BTW). But you're right, you can dub in anything for Threepio to say to clarify things, and they did.
To answer the question, I'm with the sloppy writing crowd. However, for my Stan Lee No Prize, I submit that Threepio did know, and was blabbing about it until Artoo beeped at him.
THREEPIO: [in blab mode] ...what with the Rebellion and all.
LUKE: You know of the rebellion against the Empire?!
THREEPIO: [still in blab mode] That's how we came to be in your service, if you take my meaning.
ARTOO WHISTLES [shut the hell up!]
LUKE: Have you been in many battles?
THREEPIO: [oh, yeah, right, keep it on the DL] Several, I think. Actually there's not much to tell. I'm not much more than an interpreter....
After being admonished by his more clever counterpart to shut his yap, all of a sudden Threepio begs off, saying there's not much to tell and he can't tell stories. From that point on he's as vague as can be while still answering his master's questions.