Because the JS-1 normally comes with 18 meter tips and can also be fitted with (optional) 21 meter tips. Naturally, the short tips put it in the 18 Meter class while the long tips place it in the Open class.

Of course, if you really wanted to, you could enter anything in the Open Class- an ASW-27 or even a PW-5 could be entered in Open. Assuming a certain level of mental derangement.

As far as the EB-29 being the "hands down" Open ship, the handling and roll rate definitely hamper this marvelous aircraft in weak conditions. It smokes when the cloudstreets align and running outweighs grinding around in tight 4 kt thermals.

I will take one of each (except the PW-5) when I win the Lottery.