If you look at most items even if you calculate what you do want it's mostly calculate to accumulate just over what you could buy so you'd have to spend more money.
e.g darkmeto/metowing = 2275 zc, buying 2k zc gives +100 zc = 2100 zc meaning just under yet again. Unless you have leftover zc from previous purchases or wanted to waste time on the earning free zc this would mean you'd have to spend an extra $10 on 1k zc when you only need 175. or like metorium head that's also 1,001. everything is really just over