I will offer the following until a better answer comes along.

Continental and other partners get seats at low fares from United to sell. United has sold the lower fare seats that they kept but Continental still has some left.

Flying LAX-LAS, a lot of people wouldn't consider checking CO becasue CO doesn't fly the route - they just go to UA or another carrier that does and buy there. That is my thinking about why CO still has the lower fare seats.

Did you check fares on US?

About 9 years ago I did a search on United for a fare from IAD to Germany. The results showed UA and LH. One of the LH fares was lower than UA's and then I noticed that the departure time was the same as a United flight. I ended up buying the LF codeshare flight, operated by UA, at united.com for maybe $100 less than the UA flight. I got an explanation at ft for this that was something like what I wrote above.