I can explain it by watching the movie.
They knew there was a signal of non-human origin that they wanted to investigate. They put Ash aboard to make sure it got done. The Special Order also said they wanted the ship to bring back a life-form, that's it. It doesn't specify what. For all the film presents, the Company may well have wanted a sample of whatever it was that made the signal.
If they knew about the Alien, capital A, it makes no sense to send a tug ship. If they knew the signal was from a Space Jockey, or Engineer, or whatever, then considering Prometheus, it also makes no sense they'd reroute the Nostromo. In either of those cases, they would've sent a dedicated team to investigate -- which is exactly what they do in Alien3 when the Company knows there's an Alien on Fiorina. They didn't just nudge the nearest Meals on Plasma Engines in its direction.