The caveat is that lots of categories are like the category "single-celled organisms," from what I can tell. I would guess that you are familiar with mathematics, so you can look at math and go "wow, there are so many sub-fields. Other fields aren't like that!" Meanwhile someone familiar with biology might think "wow, there's so much to study within biology; math doesn't seem like that!"

Edit: on the other hand I could be wrong - if you're not particularly familiar with mathematics research compared to biology research my story is inaccurate.