Ahh yes now i see. But yes the reason is simple the translation is applied concurently to the location. you can do this however. By grouping and rotatimg that group and then the translates channels are in fact slanted because the parent turned the cordinates.
But i suggest you do a path animation and then adjust the curve because now it is normalized and constrained i one axis thisway no matter what way you turn the path to go the same curve affects the thing. And the speed is just a function on how slanted the curve is (woohaa a integration node would be nice),
Now you can then proceed and calculate a approx speed curve of this and show it on the graaph editor if you wish... atough you cant use it to edit the thing.
you could offcourse do aplug for this but cordinating it behaivour would be hard to controll even in that case... and the usage problematrics would be almost as complex as having two groups
but a corve node that could derivate or integrate would be super usefull tough!