SUMMARY
Adding a category adds it as a subcategory of what's selected, which is confusing.

STEPS TO REPRODUCE
For example you want to add a subcategory of Bills (one of the standard categories) called Internet
1. Open Skrooge, click Categories
2. Click Bills, then click Bills > Fuel Oil (I think this is one of the standard subcategories)
3. Type "Internet" in place of "Fuel Oil" in the Name line.
4. Click the + add button.

OBSERVED RESULT
The Name line displays Bills > Internet , but the new category is created as a subcategory of what you had previously selected, i.e. Bills > Fuel Oil > Internet.

EXPECTED RESULT
The Name line should match what category is created, so if it shows Bills > Internet, then that's the hierarchy I would expect gets created. However, that would make it hard when you want to create a subcategory of something that doesn't have any existing subcategories. Hmmm... hard problem!

SOFTWARE/OS VERSIONS 
Linux/KDE Plasma: 
(from flatpak)
KDE Plasma Version: n/a 
KDE Frameworks Version: 5.59.0
Qt Version: 5.12.4 (xcb)

ADDITIONAL INFORMATION
The workaround to unexpected behavior is to drag the newly-created category where it belongs.

The Skrooge Handbook https://docs.kde.org/trunk5/en/extragear-office/skrooge/categories.html should explain the Add behavior, and should mention that you can drag categories into and out of subcategories to rearrange them in case the new category isn't where you want it in the hierarchy.

The Add action for Categories uses a different icon and layout and has no help or tooltip (Bug 409703); maybe its help/tooltip could dynamically show the result of clicking it, e.g. "Create new subcategory of Bills > Fuel Oil").