I could be wrong but there's no mechanism in the HL850 to adjust blade head tilt so far as I'm aware?
To me it sounds like there's some wear or possibly damage causing a difference on DOC, having said that though, it's less than 1mm so I'd be jumping at buying it at that price.
It's also quite possible he's maybe not keeping even pressure, producing a slight difference. Either way, it's the best planer ever made IMHO. And you can buy spiral TCT insert heads from CSP Tooling very cheap. Mine was around $120AUD all up I think.