Well there isn't anything weird with the counting or mathematical usage, just the names for them seem not to use a typical decimal system. For example, in English and Norwegian, 75 and 82 are straightforward compounds of "7,5" and "8,2". In English, seventy-five, eighty-two; in Norwegian, syttifem, åttito.

But in Danish, 70 isn't syv-something. Instead it seems it's halvfjerds, which sounds like half a fjerd, except a fjerd is not a number. ;-) And even more confusingly, 50 sounds awfully like half-60.

From what I've read, it used to be "half four twenty" (halvfjerdsindstyve), where "half four" is an archaic way of saying 3½, so 3½ 20 = 70. Which later, I suppose, got contracted into halvfjerds. Admittedly you can just treat them like opaque symbols to memorize, so I doubt anyone thinks of 75 as "5 and 3½ x 20". But the Norwegian numbers seem more orderly...