Both standard and keyway end mills are generally manufactured to the same geometry features such as rake, clearance, base material.
Even the blank dimensions and coatings applied are the same. The difference is the tolerance of the cutting diameter of the tool.
Most standard single end, end mills are manufactured with a plus tolerance from its nominal size. This, coupled with a slight back taper over the length of cut, helps to compensate for deflection and provides additional tool life.
On the other hand, to produce the proper slot width for standard key stock, a keyway end mill, manufactured with a minus tolerance from its nominal size, should be used.
The minus tolerance allows the tool to mill a finished slot as close to nominal as possible, insuring a slop-free or light press fit of the key.
If there is excessive over-cutting of the slot when using a standard keyway end mill, particularly when milling on older equipment, undersize keyway mills are also available in 0.001 increments to compensate for that.