While every human world has a distinct character (some more distinct than others) they are derived form the same base stock. Not only do all humans originate from the same Terran gene-pool, but the original human colonists built their homes using STC technology. This not only ensured a degree of social and technological similarity, but actually meant that many of the worlds settled were transformed into places pleasing to the human form and human aesthetics. Vast terraforming projects taking millennia to complete turned many worlds into copies of Earth, or as near as one could get given local limitations. On other worlds the effect was not so overwhelming, but adapted Terran strains would mingle with manipulated local flora and fauna. Even where no terraforming could be undertaken did STC technology dominate, giving everything a distinctly Terran feel.

During the Age of Strife much was lost, but every world still had the same roots. And with the arrival of the Age of the Imperium every world be reunited with their Terran roots. Sharing not only a common ancestry, but also a common destiny, the worlds of the Imperium may be diverse, but they are still surprisingly similar. The hardy grox, a genemod strain of an Old Earth herbivore can be found almost every Imperial world capable of sustaining it. Local mutations have optimized it for local conditions, but it is still the same animal that left Terra all those years ago. Imperial Gothic architecture might be all the rage in the 41^st Millennium, but it and older styles are still all based upon STC building templates. Various cereals are still grown under distant suns, covering sometimes covering continent-spanning farms.

Even on worlds that are reunited with Terra in the 41^st Millennium, possible after having endured alone in the night since the Dark Age of Technology, will have elements that are recognizable Terran human. There will be traits in the culture, in their languages, in their technology (if any) that can be traced all the way back to the original STC culture that settled these worlds so many years ago.

This has one really GREAT effect for the DM! He doesn't have to completely re-invent everything for every single world of the Imperium. There is actually a at least fairly reasonable reason why human grow corn, herd grox, and live in hive-pyramids regardless of what planet they live on. So knowing that the GM can focus on the important and/or interesting differences rather than focus on cereal production :-)