Err both manufacturers historically use both kinds of numbers at different times...
This last time MS may have well been being disingenuous. It probably worked. "7 million to 5 million, not that much difference".
As a practical matter I imagine it's easier to use sold in. You'll have easy internal access to those numbers. Sold numbers, I am not sure how complete or up to date a picture they necessarily have. There's not even trackers in many of the smallest markets I assume.
I should or could also have said "sold in". As it's a stricter definition than "shipped". Shipped can be anything off the assembly line, sold-in is sold to retailers. Sony used to use the looser definition, but a few years back they all aligned to the sold-in definition, which is good for comparison purposes.
But Sony are now quoting sold numbers - as in customers hands. And not the sold in/shipped numbers that MS are using. It makes the MS numbers look artificially higher than they are in actual fact. Compared to the Sony numbers.
* Sony are saying they have 7+ million PS4s in customers hands and I guess from the MS shipped numbers they might have ~4.5 million in customers hands!?
It's a disingenuous little trick that Sony used quite a lot with the PS3 to start (and still do with the Vita and Phones).