Racks and controllers question

Why is it that one controller needs to be assigned in the rack edit section itself before working and another works simply by adding it to the top of the plage input section?

For instance, In the top section I have as inputs the SL88 as the keys controller and the Korg NanoKontrol for other control.

Now when I open the VST for editing in the lower section the VST “sees” the Nanokontrol but does not see the SL88 unless I ALSO add it in the bottom section RACK: Midi In - As I wrote the Nano needs no such entry to be seen by the VST…why is this? It’s just another thing that makes the racks setup confusing…