It is a bit odd. But I think it's either that:
1. the 22MP is something entirely made up and not based in reality, someone was ooh look 3x3 bin for video I bet it will use that and made up all this 22MP, 7fps stuff and sent it in and everyone else copied
2. it will be 22MP but the 1DX didn't use it because they felt they needed to hit 12fps (14fps without mirror) and digic 5+ couldn't quite handle it or maybe it added just a bit too much extra heat to handle it or maybe some new ADC readout from the best low ISO DR couldn't run quite that fast with their tech, etc. OR maybe some types of pros were already complaining that 18MP was annoyingly large to deal with and they were afraid to push it even a bit higher without having them totally flip out on them and yet all the same Canon really did want 22MP badly enough for certain video reasons so they thought it worth it to spend money making two different sensors even though they are so close in MP?
It does seem odd to me they would go to the expense to make a new sensor when they are seemingly so close though. Then again the 5D did use it's own sensor and I guess 12.7 and 16.2MP are somewhat close too but I thought Canon was happy with the savings from re-using the 1Ds3 sensor but maybe they simply felt something about it important enough on the video side of things???
All I know is that every tech paper I have ever seen strongly, strongly hints that any loss in high iso performance when the MP counts are so close (18MP vs 22MP) wouldn't really be noticeable to the eye, just some modest fraction of a stop, so I think it has to be due to some other reason.
Let's go further.....if we are to assume the 5Dx has a single Digic 5 to support 22Mpix at 7 fps and a single Digic 4 dedicated to the AF per the 1Dx specs (as you have stated you believe the 1Dx AF will be in this camera), then the two Digic 5 processors in the 1Dx should be able to handle 22Mpix full RAW at 14fps.
Although no dual digic body, so far, has ever had 2x the throughput as the single digic models, the scaling might not work quite so well due to various little things to let them get the full 2x.
But they didn't do that. They put an 18Mpix FF sensor in the 1Dx, not this rumored 22Mpix FF sensor. If these are so close in image quality that nobody can see a difference as you state, why then is it not in the 1Dx?
As I speculated above.
And why would the 1DX need better sensor performance? It's more of the high speed above all else cam.
Gets you thinking, doesn't it?
Regarding your last statement, of why the 1Dx would need better performance....because Canon clearly said in the 1Dx press release it was a merging of the 1Ds and the 1D to produce the best camera possible. I seriously doubt that Canon wants to put an IQ crown on a camera that is not the successor to it's 1Ds.
In effect, it would be repeating the same thing it did when the 5Dmk2 came out....steal the IQ crown from the 1Ds and put it on a camera that was not in it's 1D pro line. A camera, mind you that did not have weather sealing, had 3rd world AF and metering, and a whole host of other features missing.
Does Canon what to put it's IQ crown on a camera that costs $3k or on it's flagship camera that costs $6800? For sake of argument, if the new 5Dx has better IQ than the 1Dx, has the 1Dx's AF and metering, and shoots 7fps, is there ANY reason to by the 1Dx for studio use? Or wedding use?
I can't think of a single reason (lone exception is if you shoot a lot of outdoor weddings and want the weather sealed body).
Again, I hope it's all true on the rumored specs and it is all that (and more) for $3k. I just have a hard time believing that, knowing what I do about marketing, etc. Canon would be guilty of telling the world "The 1Dx is the best we can offer, $6800." and then turn around 6 months later and whip out a 5Dx with 85% of the 1Dx features and better IQ at $3000.
That is a tough sell to me.
BTW, thx for a really good discussion on this, much appreciated.