probably because that area (top of the head) is more sensitive to andogrens, and there is a correlation between dht and the sebaceous glands. i have something similar (dandruff) and the only affected area is the top of the head, sides and back are fine and even feels much less oily.
Means increased androgen activity at the hair follicle and enlarged sebaceous gland. Bad for hair. I never had oily hair before male pattern baldness kicked in.
I had that very same issue before, where my scalp was really oily only on the top along with losing a lot of hair and not on the sides. I don't have that problem anymore since getting on Dutasteride. Getting on treatment can help with the oily scalp issue.
Dutasteride because of the type 1 suppression. I got massive shedding and scalp issues using finasteride. Take baseline blood work so you can figure out the culprit if you experience any issues and are using multiple treatments.