Are these valid reasons for being unavailable?

I recently started a new job and filled out an availability form. It asks me for my availability and reasons for why I am not available at a certain time. I thought this was weird because no workplace has ever asked me why I am not available.

(I put Sunday as a study day, family commitment 2 hours Saturday morning, and school Monday to Friday from 8am-11am. And available all other times till 10pm.

After I submitted it my manager requested a meeting to go over me form.

She told me that my reasons for my unavailability are not reasons the company excuses as unavailable.

Have you ever been in this position before? Can a workplace really tell someone what is valid or not??