Odd pedal wear (and knee problems)

So I bought another bike for winter in October. I moved my pedals from my existing bike onto it, and bought new ones for my existing (summer) bike. Both the same pedals (Look Keo Classic). I only have one pair of shoes so irrelevant.
After a few weeks riding the winter steed, I started to notice my feet were riding very pigeon toed (ankles out, knees in). I have my name on my frame (sad, I know) and noticed that this was wearing off - my thighs rub my top tube, however my ankles stick out. I measured every detail to match the summer bike, and they are exactly the same, cm for cm.

I made a few basic adjustments as I was feeling pain in my knees on return from a ride, like adjusting cleat position to try to bring my knees and lower leg in. A lot of people advise against this but I did the dangling of legs trick and my feet seem straight with my legs, I don't walk funny, and the key piece of info - on my summer bike, my legs and feet are straight.

So for months this has gone on, now we're up to 5, and today my foot actually started slipping out the pedal involuntarily (this is all on the winter bike, really). I fitted new cleats a few weeks back, so it can't be this, so it got me thinking. I was just looking at the pedal when I noticed it looked as though a piece had snapped off. I checked the other pedal - this too had a piece missing so I although it looked odd, I considered it could be as designed. When I got home I checked against the other pedals which have had 500m on them max, and the attached picture shows what I found.

Obviously I am very pleased I have found the source of my problems, however this doesn't help me. I obviously have an issue with wearing away the pedals in this manner - can anyone suggest what may be causing this? I am considering seeing a specialist but I will obviously get some new pedals first and see how that works.

Anyone seen this before? It shocked me quite a bit as that is a lot of wear.

For the record, those worn ones have probably done 5500 miles.

I'd appreciate any opinions on this.

cheers