The B-airs of Dedede and Kirby have deceiving range and have lingering hitboxes where as Marth's is a short and precise hitbox with decent range. There is no such thing as priority in the air, in the air its only a matter of which hitbox hits whose hurtbox first. Often times we hit eachother because my B-airs aren't disjointed and I throw a part of my hurtbox right into your F-air.
In the air, its all about spacing. There is no priority.
A guide to priority:
Priority
Priority is what determines if any one attack will beat out, lose out, trade with, or clank with another attack. The player closest to player 1 has the most priority, although the instances where this matters is extremely limited. There are two types of priority: Intrinsic Priority and Implied Priority.
Intrinsic Priority
Intrinsic Priority is what the developers added for ground and special attacks that directly compares which attack has more priority. If one attack has more intrinsic priority than another attack, the first will beat out the second, completely overriding it. If the two attacks have the same intrinsic priority, then the two attacks will clank, causing both characters to return to their default state. This only applies to ground/ special attacks, and only when two hitboxes from these type of attacks collide.
Implied Priority
Implied Priority is a function of how far the hitbox of an attack extends past the hurtbox of the character performing the attack (aka, the level of disjointedness of an attack). This mainly applies to aerail attacks' interactions with other (including other aerial) hitboxes. Since aerial attacks don't have intrinsic priority, when a hitbox from an aerial overlaps another hitbox, the priority is not determined then (i.e. Aerials cannot clank). In this type of collision, the "winner" is determined by whose hitbox overlaps the other's hurtbox first. The farther separated your hitbox is from your hurtbox, the more likely you are to hit them out of their attack and less likely to trade/ get hit out of your attack.
Grab Priority
An interesting thing about the priority of grabs: When a grab connects on the same frame as an attack done by the grabber, they "trade". This type of trade results in the grabber taking the full damage of the attack, but no knockback, and grabbing the person.
Video by Magus: http://www.youtube.com/watch?v=GeuXNw5P3G4
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