Hey Wyatt,
It was a design choice. Any quests that don't fall under the typical Main or Side quest types show under "Other".
Reason being is I've no idea how many quest types you might make and I can't make the menu accommodate for them all if you decide to add 20 different quest types for example.
To compromise, the "Other" category option is shown as an umbrella for all custom quest types. Additionally, so you can tell what quest type a quest actually falls under, the quest title is appended with the quest type too as well as being displayed in the quest type's colour specified in the plugin's user interface.
Hope that answers your question :)