[Hui]

I've not studied it yet but the reason could be because one leg does rebound and the other takes care of compression. ie. the internals would be different.
I doubt very much if it's anything to do with the mount for the calliper, it's not hollow.
Years ago Honda used a valve triggered by the movement of the calliper to control diving under braking which changed the oil quantity slightly in each leg. It was a rubbish idea and soon dropped.

 

[Hui]

I realise it's not hollow, which is why I suspected it may have a reduced capacity compared with the right leg. I've read nothing of compression in one leg and rebound in the other. It was mainly Marzochhi forks that had that feature, but wasn't particularly successful. The fork leg without any compression damping tended to wear the bushes under hard use. I know of a few riders who had such forks replaced under warranty for persistent knocking noises from them.

"It was mainly Marzochhi forks that had that feature" ?

WP Suspension are doing it currently and are as good if not better than Ohlins. The theory being a fork with one thing to take cars of has an easier time and can do it's job without compromise. The spring does the preload.

"I realise it's not hollow, which is why I suspected it may have a reduced capacity compared with the right leg" ?

Why would the calliper mount affect the oil capacity if it was solid ? Especially if the other side has no mount.
The other question I would ask is, Why would a fork leg without compression damping wear bushes ?
There would be no need to write about it in the manual etc. as they're not adjustable anyway.

You did say your guess would be...
All I did was disagree with you guess and offer a more plausible technical explanation. No need to start flexing your muscles and trying to prove your technical prowess with guesses.