You're seeing the results of font fallback, which substitutes something other than the original font you specified when you try to render characters that aren't in that font. In this case, the kana and several other characters (Won sign, others) are not present in Courier New, so you get some other font whose metrics do not match those of Courier New.
There's no simple solution to this, particularly if you expect to be displaying a wide range of characters. What you could possibly do is set up a filter, as many hex editors do, and just show a '.' or similar for anything non-ASCII (or in this case you might be able to do a little detective work and set it up to show '.' for anything that is not present in Courier New font).