Hans Wegner Valet Chair  

Many reasons...
Mass produced or rare small production.
The date of the auction...
You will find auction listings dated 2001 or earlier.
Take note of the printed copy with the listing.
Also something as silly as the cover of a prominent magazine
will effect an auction price.
The Valet chair i think was reproduced recently?
That alone gives it recognition? Hence an original becomes more
valuable. May be i just made that up. Not a dealer here. But i've
had the same experience researching one of my chairs.

It is totally random. It is...
It is totally random. It is only worth what someone at that particular minute is willing to pay. I wish there was a better answer.
I sold a vintage marshmallow sofa for 30K. The year before it reached 139K!!!

Sucks you weren't on the...
Sucks you weren't on the other end of that Whitespike (not that 30k is anything to scoff at).
I know much of pricing is complete randomness as I follow auction results closely but I guess I was more interested in why the auction ESTIMATES from the same auction houses on this particular piece vary so wildly on pieces that appear to be the same vintage/condition/etc.

Material?
I believe Wegner wished this to orignally be made of pine with wenge details, but that production versions were made of teak and maple due to strength and durability. Perhaps the high ones are for a rarer wood type.

Just stumbled across on eBay what appears to be a vintage Valet chair knockoff. Actually, I'm not sure whether "knockoff" or "heavily inspired" is the correct term, as it is not trying to be identical to the original.
I was not previously aware of vintage replicas. It's not nearly as refined or elegant as the original, but certainly interesting to look at. Here are a few photos.

cdsilva, that is interesting!

Don't forget the show-off factor. It might be a chock to some readers, but some people actually buy designer furniture to impress their guests (Nooooooooo, Really?, Say it ain't so, buhuuuuuu). Well, that's how it is.
The problem with the valet chair is of course, it's meant to be where you put on your clothes i.e. for most people, the bedroom. You can't casually put in the living- or diningroom as it doesn't fit in. If you try, your guest will think that you are not just a show-off, but also a complete idiot.
You could of course try to put it in the bathroom, but I wouldn't recommend that, as some guests might mistake it for some kind of fancy wooden bidet.
So the conclusion is clear. The only buyers for the valet chair, is people who entertain their guests in the bedroom.
(I know. I went a little off topic, but I was on a roll. Peace V)
 
@cdsilva
My theory is(theory=finer word for guessing) that the, now, seller of the chair, had to make a quick decision when he saw the chair, and bought in the hope that it could be Wegners valet chair.
Wegners design is not unknown to the seller, as he has two CH23's in the background 🙂

Upon closer inspection, I'll retract my "heavily inspired" option. Looks intended to be the same, with the seat cushion added afterwards. 
 
I won't claim to know what the seller is thinking or how they acquired it. But the listing has minimal words and is phrased carefully to avoid outright misrepresentation (i.e. avoids reference to Johannes Hansen). One could argue that this piece is "Hans Wegner Designed". 
 
And once I started poking around more after my initial post, I did come across other valet chair knockoffs. Although most of them appeared to be more recent knockoffs.