Gmail’s interesting auto label nesting behavior

Use Gmail? Create a label called “top/middle/leaf”:

You’ll that Gmail created this as a single, “non-nested” label:

Now create another label called “top/middle”:

Something amazing just happened. Now you have a label called “top/middle”, with the label “leaf” nested under it.

And for some more fun, just create another one called “top”. Now you have the whole hierarchy top/middle/leaf as nested labels.

Any idea why Gmail is doing that?