As always, polls are good for large samples and bad for specifics (as I learn each and every time I post a poll and I'm told how it is flawed).
The poll is flawed

.
Not really... and not in a bad way, but I can't accurately represent myself.
I put "Changes relative to PC's (but lower)". But that's not really the case.
First, it depends on the campaign. I ran one campaign that was all over the world...there were highly variable levels of NPCs. I ran another that was more "location based" and the NPCs were on the border of hell (so they tended to be higher level than one might expect farmers to be...because they were seasoned soliders.
But, overall, "Changes relative to PC's (but lower)" is relatively accurate, but not for the reasons most might assume.
It's not that they change because of the pcs. It is true that they HAPPEN to change relative to pcs because pcs go from rural farmland fighting badgers and goblins to urban terrorville fighting the hordes of demons that are plaguing that city (because of macguffin x and plot point 1.7). I guess I'm saying that players naturally move away from "small town boringplace" to "scary city-terrorville" as they level because of increasing challenges and rewards.
But, for my games, it's not that the players are level 7, so the npcs must be level 5-9, in true bell curve fashion. It's more that the players are level 7 and want to go where the 5-9 NPCs are, because the challenges there are substantial, but not overwhelming, and the rewards are good enough to support those challenges.
I don't believe in a level 20 guard....unless that is a level 20 guard at the gates of the citadel in the city of brass, protecting the [epic]sultan... then, yeah. I do believe in it. What I've tried to describe is this in a more mundane/cross level fashion.
EDIT: But on the other hand, it's fun to mix it up a bit. There might be a level 10 retired adventurer with level 1 guards somewhere. There might be a level 3 Aristocrat holed up with level 9 guards in a major city. It's good to make sense, but it's also good to not be a slave to balance. Sometimes great stories can occur from (at level 15) "I shoot the prince. I do 7 damage." "Ok, the prince is dead. Now what?"