I would guess that he's trying to reduce uneven way wear but I think he's kidding himself about being that carefully centered making any discernable difference.
If it did, what about making sure you always take a same length cut traversing left for every cut you take going right. For a variety of reasons I expect my ways to wear unevenly but that will just be one thing to deal with if and when this machine is rebuilt. I know absolutely and for a fact that I'll find barrel and hourglass wear simply because movement isn't always balanced, and having a table overhung from the saddle is just a feature of the design of the mill. The additional work required to correcty asymmetric wear pales in comparison to the years of inconvenience and high blood pressure from trying to eliminate the possibility.
I'd class that one with just some of the folklore everyone maintains inescapably. I know that some of the things I "know" to be true really aren't. If I knew exactly which things those were I'd correct them now. The problem is, I don't, so I just have to stay open to weigh what I see and hear and change my mind when it seems warranted. Maybe that's mental gene repair.
"Opportunity is missed by most people because it is dressed in overalls and looks like work." Thomas Edison