MMB upgrades: logic?

Does anyone know, or think they know, the logic behind why or whether upgrades are offered for sale in your Manage My Booking page?

I have an economy reservation and there is TONS of PE availability on both legs, but the system does not offer the upgrade. If I do a dummy booking, I am prompted to upgrade for either or both legs. (I was offered it when booking myself -- stupidly I thought, I'll do it later... B) )

There is plenty of UC availability too, but I am only offered the upgrade for the return. However, I am also offered a round-trip UC upgrade. ?|

As a final complicating factor, I am invited to bid for a PE upgrade on the VS45. Could that be preventing the system from proactively offering me one for cash? I could understand that.

Also, does the purchase of exit row or extra legroom seat stop the system offering you an upgrade?

One thing that I DO know is that if you use miles to upgrade a leg, the system immediately stops offering you anything for the other one.

Questions, questions.... Grateful for any thoughts.

I was going to say perhaps it was the class of ticket but if you are being offered the chance to big for an upgrade that can't be right (can it??)
I would say a call to customer services is called for. Make sure you have coffee / tea and a comfy seat first...

logic + VS = does not compute !

thanks for the replies.

I should have been a bit clearer -- I am not really interested in my own example (which was just a particularly confusing one), more the general principles. I take tonty's point about logic but unless a human is doing this on an ad hoc basis, there must be some rules, and it shouldn't be hard to deduce at least some of them. For instance, I believe I am correct when I say that if you upgrade a leg with miles, you will not see upgrades offered for the other leg -- so that's a bit of torchlight to illuminate the elephant...

This detail probably isn't very interesting to many, but I kind of wish we could develop a bit more understanding of this kind of thing. Virtually the only thing I like about the BA FT board is the astonishing depth of knowledge that some (OK, a few) people have about fares, upgrades, and the BA IT systems.