In the ideal case, you'd only need to hash ~17000 * 2^24 MAC addresses to have a complete table, but the reality is that there are a lot of manufacturers who assign MAC addresses in products using OUIs that they haven't registered. (Registering an OUI costs US$1885, and when you're a factory working on razor thin margins, that's a lot of money.)

As an example, a $17 802.11n travel router I bought from Monoprice a few weeks ago uses 00-B0-C0 as the OUI, which has no corresponding entry in IEEE's OUI database. In the past, I've purchased cheapo no-name PCI Ethernet cards (usually with Realtek chipsets and lots of empty pads and/or through-holes on the PCBs where capacitors are supposed to go) which had similarly unregistered OUIs in their MAC addresses.