What is head of a bone?
Here is a head and a neck:



Anatomy is fairly straightforward. regarding individual long bones, "heads" are curved ends of bones distal to a narrowing called a neck of a bone. Take the radius:



See how the radial head (curved) is just distal to a narrowing (neck)?

The ulnar head, though, is at the opposite end, because the proximal end isn't rounded, but the distal end is.

Metacarpal heads are distal (see blue area):



The humeral head is probably most obvious:



So, the head of a long bone is not determined by the position (it can be proximal or distal) but by the contiguous shapes.