You are viewing the pic without alpha-channel.
Switch to alpha transparency and you'll see how the texture actually looks like.
And it seems the reason for your problems is that characters on the left half are on white transparent background, while characters on right half are on black transparent background.
Meanwhile, vanilla font has all characters on white transparent background.
Strictly speaking, transparent is transparent and it shouldn't matter to Warband what RGB color transparent pixels actually have. But apparently it does matter. As I said, pretty nitpicky.