tankoubon oddness

Looking over Gintama manga, scanlations and original raws, I noticed something odd. The scanlations I have are from the SJ chapters, while the raws are scans of the tankoubon - and they're different. The tankoubon raws have some extra pages - so far I've noticed 4 extra pages at the end of the Tama Virus arc, and one at the Character Poll arc. New art, new dialogue - did Sorachi expand them a bit for the tankoubon, or what? Also of interest, at least in the Virus arc, the added pages match the anime's end - the tag with the Leukocyte King fighting is in the tank. but not the scanlation. And the final lines before that are different as well (specifically the scanlation is missing the Leukocyte King's line about being a copy of a troublesome man who can't even say thank you or cry, and what would Gintoki do in this situation? And Gintoki's "Smile." (which exchange I <3, a lot.)) This doesn't seem to be a translation error, since the page layouts are different... (The line they have instead is also cute but rather different; wonder what the original Japanese was...?)

Or is this a common thing with manga and I've just never noticed it before? I know that Saiyuki's chapters were sometimes published with incomplete art and completed in the tankoubon, but that was a different situation, so...? OTOH, given the extreme deadlines weekly shounen mangaka operate under, it wouldn't surprise me...

(This is also making me curious - when looking at some of the later scanlations I've noticed translations that are completely different from what was said in the corresponding scene in the anime - and when I've looked at the raws, the anime lines seem to be what was in the original manga; I was thinking the translation was either really bad or was possibly being done from another source (like the Chinese version), but now I'm wondering if Sorachi actually adjusts the manga scripts when they get collected in the tankoubons, and the anime is done off the new scripts?)