It's not uncommon for the mechanical range of travel and the electrical range of travel of a potentiometer to vary significantly. For example, in the 251B12T104A2NB model referenced, the mechanical range of travel is specified as 180 degrees, whereas the electrical range of travel (sometimes called the effective electrical angle) is 50 degrees.

The datasheet shows how to decode the part number to find the electrical range of travel, indicated by the number in the third-to-last position of the model number, in this case 2. However, the datasheet on the Mouser site seems incomplete and doesn't have 2 as a valid value in that position. Digikey have a more complete/up-to-date datasheet here: https://www.digikey.com/htmldatasheets/production/2758000/0/0/1/251-series-datasheet.html

In positions outside of the electrical range of travel you'll generally not see any change in resistance, as the wiper is likely in contact with one of the terminals.