Anything organic: VMs, PPDs, some friction modifiers, yellow metal passivators. Also anything masked by something else. Example: if you have something which is characterised by its P content, it might get masked by ZDDP.
VOA gives you virtually no information about additives. For example, you see calcium and you think "there is a calcium-based detergent in there". But so what? Which one(s)? There is often more than one detergent in a given oil. You can't distinguish between a sulphonate, phenate and/or salicylate from a simple VOA. And they offer different performance, so simply focussing on "Ca" and/or "Mg" is really too simplistic.
Another thing could be misunderstanding or assumption on what an element is there for. Eg: Mo - many see Mo and think MoS2 (molybdenum disulphide). But it could be in as MoDTC (molybdenum dialkyldithiocarbamate).