The reason you would have seen that was you had previously set the velocity higher with the higher kernel then when you dropped the kernel the velocity was above what that kernel could deliver.
The way it works is this
25KHz kernel means that you can pulse at a max of 25,000 times per second, if your steps per unit are say 1,000 then that means your velocity can be 25,000/1,000 = 25 units. BUT that is per second so you would multiply by 60 as the Velocity you enter is per minute, so that would be a max velocity (at the steps per unit of this example) of 1500 units per minute.
Using the same Steps per unit but 35KHz kernel it would be 35,000/1000 x 60 = 2100 units per minute
For 45KHz its 45,000/1000 x 60 = 2,700 units per minute
Etc etc
Hood