There is a difference for sure.
a On-Off-On will connect the centre lugs with either side in the On position, but will connect to neither when Off.
A On-On-On switch is often a A-Z-B switch meaning that in A and B it will connect the centre lugs to side A or B. In Z (centre position) it connects one middle lug to side A and the other middle lug to side B.
I'm not sure but I think there are different versions.
the pic below is actually missing pins in the ON-ON-ON graphic. should be 6 pins to and the centre pins are usually not connected, iirc.
