what is the difference between these statement

[Tim Rentsch]

I'm a bit confused about why 'EOF' can't be 0x100.

Do you have a copy of N1256 or N1570? If not grab one or
both:

http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf

I know it can't be any value of 'unsigned char' converted
to 'int', that is a valid character at least.

The bold entries in the Index are primary or definitional.
For 'EOF' that will lead right to 7.19.1 (in n1256), which
says EOF is an integer constant expression, with type int
and a negative value. (And 'int' will always hold 0x100
as a positive value, in every implementation.)