[skidlids]

Do not confuse Fork travel with the overall travel of a progressive spring

A progressive spring could go from 0.6kg/mm to 1.2kg/mm over a distance of 180mm

So has an average spring rate of 0.9kg/mm

Lets assume the rate of change of the spring is linear (gets more complicated if its non linear)
Now if your fork only has 120mm of travel, without any pre-load the travel will use the 0.6kg/mm up to the 1.0kg/mm part of the spring, giving you an average of 0.8kg/mm, so softer than the spring average.
Now if you pre-load the spring by 30mm you end up using the 0.7kg/mm to 1.1kg/mm part of the spring giving you an average of 0.9kg/mm the same as the spring average.
Now if you pre-load the same spring by 60mm you use the part of the spring that is in the 0.8kg/mm to 1.2kg/mm giving an average spring rate of 1.0kg/mm resulting in stiffer than the total spring average.