[Ice fyre]

Now I'm not sure about this. But how long were the colonists on this world?

It seems to me that it would take time for the atmosphere converters to work. The planet we saw in Aliens is very different to the one the Nostromo left.

I would surmise that they would have had quite a bit to do before they explored this world fully. The ship was a fair way from the colony. Also maybe all the colonists wouldnt have gone down at first lack of manpower and the risk of an unteraformed world would have had them staying close to their bases.

It seems to me it would worked like this. They would have had a skeleton staff go in and build the atmosphere generators, now I'm assuming there were a few not just one.

Next the second wave of colonists would go out a few years later the planet being more habitable by then able to support a colony less expenisivley (no need for expensive specialised atmosphere equipment like sealed domes and such like) there is also another reason they waited so long. Perhaps they were told to leave that area alone or misdirected by company teams amongst the colonists.