The Oread race seems to become venerable at 250, but they live for another 6d% after that? Does that honestly mean they could be venerable for almost three times as long as their entire life prior to that? Or am I reading the table wrong? What's up with that?
An oread's unusual lifespan is correct and likely due to its extraplanar descent
The Advanced Race Guide says
Oreads are humans whose ancestry includes the touch of an elemental being of earth somewhere along the line, often that of a shaitan genie. Stoic and contemplative, oreads are a race not easily moved, yet almost unstoppable when spurred to action. They remain a mystery to most of the world thanks to their reclusive nature, but those who seek them out in their secluded mountain hideaways find oreads to be quiet, dependable, and protective of their friends.
So an oread's unusual lifespan is likely due to its descent from from a similarly long-lived (perhaps even unaging) extraplanar creature, much like the ifrit is from an efreeti, the sylph is from a djinni, and the undine is from a marid.1 These races also have the same 250+6d%-year maximum age like the oread.
(With this in mind, the same should hold true of an aasimar and tiefling and, perhaps, the dhampir, yet a member of those races is only as long-lived as a poor human.2 An in-game explanation might hinge on the extended distance from the holy, undead, or unholy blood that makes such creatures different, but the more likely metagame explanation is that, in most campaigns, a creature's current hp, for example, is much more important than a creature's maximum age.)
Note that this makes the elemental-descended races rivals with elves for longest-lived race, introducing some interesting storytelling elements. For example, since the elemental-descended races become venerable (hence smarter, wiser, and more charismatic) earlier than elves do, the oldest of the oreads might have a different perspective from the oldest elf on events that occurred in the distant past, the oldest oread having been smarter, wiser, and more charismatic than the oldest elf when such events took place.
1 The description says that an undine could instead be descended from a water mephit:
…Sure. Whatever floats your boat, I guess.
2 James Jacobs mentions in this 2015 thread that the Advanced Race Guide errata and subsequent reprints changed the original aasimar and tiefling ages to better mesh with ages of creatures previously published in Paizo adventures.
You are reading the table correctly, and you may note that many of the other races have similarly rear-loaded lifespans (all the elemental planetouched, and elves and dwarves to a lesser extent). It does seem a bit ridiculous, but if you consider the availability of magical healing, the ability to flawlessly cure injuries and even serious diseases like cancers would nullify most of the normal causes of death for the elderly. Those with access to such magics could easily live for a very long time. I would argue that the maximum age posited for humans and metahumans is actually quite low when you take that into consideration, probably because it is meant to correspond to the sort of life expectancies we currently see in real humans - it really ought to be markedly higher, maybe another 2d20 or so.
At any rate, remember that this is the maximum possible life expectancy assuming that nothing else kills the person first - so without ready access to magic healing the individual would probably die of disease or injury long before that (in a particularly grim setting famine might also be a significant contributor). Plus, they'd have to get a very lucky roll on that maximum lifespan.