chitzk0i
Explorer
I think the technicality is the answer. The compendium defines speed as "The distance (in squares) that a creature can move using the walk action." and says armor "imposes a penalty on its wearer's speed", not movement or something like that. In contrast the slowed condition specifically calls out that it decreases the all modes of movement. I wouldn't be surprised to see this get changed in errata in some way though.
In the initial rules, there was a 'fly' action, a 'climb' action, and so on. As of some errata, you now take the walk action and choose a speed to use. From the rules update:
Page 47: The following text replaces “The Fly Action”
section. These rules clarify how f light works and
remove “f ly” as an action. The revisions also expunge
“crash” in favor of “fall.” See “Falling,” page 284 of
the Player’s Handbook.
[...]
Fly speed: To fly, a creature takes the walk, run, or
charge action but uses its fly speed in place of its
walking speed. A creature that has a fly speed can
also shift and take other move actions, as appropriate, while flying.