To elaborate on rpotts's post: As far as i know it's about the environment collision box, the thing that makes you stand and land.
If you stand it's as tall as the character, when you airdodge it is a bit smaller, so if you try to airdodge sideways from a standing position, i.e. try to wavedash, you will not touch the ground.
When you jump the collision box becomes smaller (especially for many double jumps where the characters curl up), and the subsequent airdodge will make it larger, so if you fall down close enough to the ground or airdodge while going through a platform at the right time you will land even when going completely to the side because the collision box will extend to the ground.
Look at any BizzaroFlame match, he'll often do the double jump before the waveland because it makes him so small which in turn makes it easier to get close enough to the ground for the perfect sideways waveland. Here you also can see him mess up quite a few in hand-warmers: