background variable always set to light

In my vimrc I have:

set background=dark
colorscheme xoria256

however when i run vim, if i do "set background?" or "echo &background", the answer is "light", not dark.

If i do "verbose set background?" the answer is "Last set from ~/.vim/bundle/xoria256.vim/colors/xoria256.vim", but if i open the file i see "set background=dark", so why &background is set to light?

what's even stranger, if i do "set background=dark" some colours change... (e.g. comments turn to cyan)

any insights on this? is it a bug?

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