Please correct me if I'm wrong... around September 2006, XB360 sold 5.9 million systems worldwide. So, XB1 is still under 6 million sold. Not that great... not that great at all.
How do you know Xbox One (or X360 in 2006) worldwide sales? Answer: You dont.
Last gen worldwide 360 shipments tended to be about double USA sales. By that rule of thumb X1 has probably shipped (not sold) ~7 million by now.
Even using shipments numbers, we can easily see X1 is almost certainly ahead. It shipped 5.1 million it's first two quarters, last quarter they combined the shipments but it was 1.1, and they haven't reported the September quarter which should be a healthier quarter. There's no way X1 only shipped 800k in the last 6 months. US sales alone easily top that during that period.
No supply constraint on launch for Xbone, but after launch it sold worse than X360.
Demand is demand. That consumer who couldn't get an X360 in November 2005 then should have picked one up in March 2006 or something. If 360 hadn't been as supply constrained in holiday 2005, it's early 2006 sales might have looked markedly worse without that pent up demand. The sales dont go anywhere, at some point the sales are the sales. By this point in 2006 360 was long since not supply constrained, and had sold significantly less through that period than Xbox One.
XB360 was released November 2005, and sold 5.9 million by September 2006... while XB1 was released November 2013, and sold roughly ~5.6 million this past September.
I dont know where you're getting this XB1 sold number. At best you're mixing and matching who knows what and comparing that to a X360 shipped number, and then comparing not like for like.
Here's what we know factually:
360 shipped (I assume), you say 5.9, I have it at 6.0 for it's first four quarters.
X1 shipped 5.1 it's first two quarters.
In it's third quarter, MS did not give a X1 number, only a 360+X1 combined number, which was 1.1.
MS has not revealed any September shipments yet. Those should come next week? Soon anyway.
It would be pretty difficult for X1 to not have shipped >900K over April-September. It's USA sales alone were about 900k in that period.
As for end user sales, well, ignoring Japan, the only ones we have from NPD show a substantial lead for X1 gen over gen.
All that said, that lead curve is flattening to some extent, but it still exists.