At runtime the animation keys use time (float seconds), not frames. I'm not sure what Game Maker is doing, but if animations have a different number of frames, they have a different duration whether it is in frames or seconds.
Where does Game Maker show how many frames an animation has? That doesn't seem like something it should be doing, unless you are using a PNG sequence.
Are your animations looping? Maybe you are seeing the walk animation play multiple times in a time period longer than its duration.
