Buyer beware indeed. The volume quoted may also be with the boot floor at its lowest – even though you would only use it with the boot floor level as a lip can be an issue. Or with the rear seats pushed fully forward leaving no legroom in the back, I think the Tiguan does this in its specs as it makes it look bigger than the competition at 620ltres instead of 500 odd.
The 900L quoted by the Outlander will be to the roof, rather than to the top of the back seat. I looked at the Outlander 4 years ago and it was quoted then as 590litres to the top of the seat. The car has been updated since then, but the body shape is still the same.
In life, it's not who you know that's important, it's how your wife found out.