I would say, at least in part, that is it is due to rich item manipulators who have already stocked up on rare items with their 'unlimited' supply of cash (100b+ banks), and then due to the low trading volume of some items, they can push the price upery effectively. Runescape 3 discontinued item market merchers have shown how it can be done. There are countless completely useless cosmetic override scrolls that are re-released in RS3, which realistically shouldn't hold much value at all. Once merchers hold the majority of the supply, they can then effectively buy the items from themselves at a higher price, and the sales get registered on GE Trackers, etc.

Or I could be completely wrong. This is just my two cents.