It's really not a mystery.
On August 12th Sony announced they sold 10m units to consumers. This means their shipped to retailer number is higher (because there are obviously units sitting on the shelf when they made that announcement). Let's assume WW there are 2m units on shelves (which is kind of low), so at that point in time it was 10m to consumers, 12m to retailers. Fast forward to Nov after Destiny launch Sony announces they hit 13.5m. We can scratch out it being the sold to retailer number unless one wants to put forth the argument that between August to Nov Sony sold no units to consumers and that same 2m units were sitting on store shelves untouched, their sales just came to a halt. In the US alone cumulatively for August, Sept, Oct they hit 1m in sales. Getting 2.5m from the rest of the world in that same time frame isn't out of the question at all. Given that it's the holidays Sony would ship more units to retailers in advance, so we'll say 3-m units. That gives the estimate that most analysts are pegging the end of the year number at 17m.
Sony's 13.5m number is obviously their estimated number sold to consumers. Unless someone wants argue that from August to end of October, PS4 sales completely cratered all around the world and as such Sony switched to shipped to retailer estimates when they announced the 13.5m number (which would also mean Sony flat out lied on their financial report that were just released...which is totally illegal).
MS' numbers by their own statement is their estimates for number shipped to retailer already accounting for the holiday shipments. XBO will take November and December in the US, but that won't be enough to offset the difference WW.