Forgive me if this sounds like a naive question, but there is something driving me crazy on the BA website.

I'm looking to book RT flights from LHR to SFO for March. I plugged in March 10 as an example to see what the fares are like.

Before I clicked on any of the outbound or inbound choices, I scrolled to take a look at what the site gave me for outbound options--two non-stop flights on BA for £310 and a variety of AA and mixed code-share 1-stop flights ranging in cost from £410 to £1126.

For inbound options, I had plugged in Sunday March 22 and the site showed me the cheapest option at £310 being a 1-stop via LAX, 1-stop codeshare options from £350-1130, and non-stop choices at £350 or £501.

So far so good.

But as soon as I click either an outbound or inbound flight option that contains an AA codeshare flight, EVERYTHING reprices on the webpage! For example, if I click the BA non-stop going out at £310, then the flight back via LAX would cost £330. But if I clicked one of the AA flights going outbound (which costs a bit more than the non-stop BA flight despite the extra stop), then the flight back via LAX prices out at £262.

And if I start with the inbound flight t £262, then the outbound flight prices al go up!

So before I choose any legs, the system is showing me the cheapest options on either leg, but it is not letting me choose both cheapest options, since choosing any one causes the price on the other leg to go up.

So is there a trick to this? Or is it just a messed up system?