Potentiometer values don't add up
This is expected behavior. 

So I rummaged around my old parts bin and found similar behavior on most old potentiometers. 

There wasn't have a big enough sample of newer ones to see a difference. 

Possible reasons for this behavior: 


Oxidation on the resistor track and / or wiper. There was one potentiometer where the resistance sum changed  wildly over one sweep, up to twice the nominal value, but became better with exercise. 
Construction: The resistor itself is a flat band that the sweeping contact travels across. If the contact does not reach across the whole width of the band, or is not correctly aligned, the resistances will not add up. 


One of the more consistent potentiometers gave its life for geekness: There were two sweeping contacts, each about one quarter width of the resistor, one traveling along the inner edge, one traveling along the middle. This means, when you measure across the resistor, you measure all along the whole width of the resistor. On the dot of the sweeper, you measure only a tapering part of the resistor, which does not have as much resistor in parallel. 

Trying to visualize: 


A-B resistance is like this: o========o; 
To the wiper it is like this: o==>&lt;=====o.
It is the taper of the current distribution that leads to the increase in resistance.


Do I care about this? No. First, we only care about the relation of the two resistances. Second, we are dealing with 10% accuracy devices. Third, if precise control over the parameter was required, one would have chosen a different tool. 

Ms. Ohm and Ms. Kirchhoff can safely put away their husband's heart medicine.