Creatures within 10 feet of the caster but further away than the last creature above are unaffected because the spell ran out of power before it ever got to them.
That's not always true, though.
Let's use a Widened Sleep, just to give us a few more squares to play with. 20' radius burst.
Let's say we have two kobolds, a gnoll, and an ogre. The ogre is 5 (and 10) feet away from the Point Of Origin, the gnoll is 15 feet away, and the kobolds are 15 and 20 feet away respectively.
First creature required to make a save? Kobold A. Second creature required to make a save? Kobold B. Third creature required to make a save? The gnoll.
Let's say they all fail. The last creature affected was the gnoll, but kobold B is further away than the last creature.
The ogre is closer than all three, and had they not been present (or had the spell not been Widened), he would have been required to make a save. But the spell 'ran out of power' after affecting creatures further away from the point of origin than he was.
So the "expanding wave" analogy is flawed, because distance from the centre is not the sole criterion of which order things are affected.
Now, under the "required to save = affected" reading, we roll saves in that order - Kobold A, Kobold B, Gnoll - and at that point, we stop, because whether they saved or failed, the spell has affected 1+1+2=4 HD of creatures.
The way I read it - based on Hypnotic Pattern's wording, that an "affected creature" is one who actually suffers the effects of the spell and therefore failed his save - we still roll saves in the same order - Kobold A, Kobold B, Gnoll - but we only count the creatures who fail the save and are thus 'affected creatures' into the total of Hit Dice Already Affected. So if the kobolds both fail but the gnoll saves, our total of hit dice affected is 2, and 2 hit dice remain; insufficient to affect the ogre, who is again left unscathed. But if all three succeed, our total of hit dice affected is 0, and 4 hit dice remain; sufficient to affect the ogre, so he must roll a save to see if he is affected or not.
Either case is still subject to the Schrodinger's Wave problem. The ogre, standing beside the Point Of Origin in an empty room, certainly needs to save...
unless there are invisible kobolds standing behind him, in which case they will soak up the magical energy and protect him, despite being further from the centre. The ogre doesn't know if he needs to attempt a save until the whole of the area of the spell has been evaluated.
-Hyp.