A possible answer to “Why?” – as I understand it, the NLA strips will supercede any Actions when NLA is enabled. So if an Action is made into an NLA strip, and NLA is enabled for that strip, it will not allow any other Actions to have influence. However, as long as the Action you are editing is the same as that which was used to make the NLA strip, the edits are reflected in the strip’s operations (in my experience at least). But if a different Action is selected, its edits won’t update the NLA strip.
If you disable an NLA strip in the NLA Editor, then whatever Action is linked to your Object/Armature in the Action Editor then takes over control, but edits won’t show in the NLA strip if it’s linked to a different Action.
BTW, because of this I’ve taken to naming Actions very specifically so I’ll know easily which is in operation, either via NLA or otherwise, at any particular time, by cross-checking the NLA and Action Editors.