Earlier quoted context omitted.
There was a secretary for the group, not for any one professor. If you'd visited in, say, 1925 the professors in the group might've had one secretary each.
That has more to do with those secretaries being replaced by computers than anything else. When everyone can type, and mistakes are easily corrected, you don't need people whose only job is to type anymore.
There's been a trend towards typing ability, yes, but hardly decisive. The old secretaries would do the bulk of e.g. organising conferences or the professor's travel, that's needs typing but not only typing.
The trend (in .no/.se etc) towards equality, against certain symbols of prestige, mattered a great deal. A long time ago having a secretary was a symbol of power, appropriate for Professor Dr. Lundman. At some point forgoing one became a symbol of modernity, and private secretaries became much less common, and the universities reorganised to provide specialised services such as ticket purchasers and maybe grant application writers instead of private secretaries.