I guess it depends on how you take it. It could stand to mean that & matches a whole word, and therefore has no meaning touching anything else, or it could mean that it matches only word characters and therefore cannot match more than one word, in which case the particular behaviour you're describing would make sense.
Of course even by that second definition mIRC's behaviour isn't consistent seeing as in the wildmasks &c and a&c the & is treated as a literal ampersand.
I suppose there's the age-old 'backwards compatability' argument for keeping things the same, personally if it does get changed one way or the other I'd like to see the second definition for & used, essentially making it equivalent to a \S+ regular expression and making things like a&c match a word beginning with a and ending with c.
