I've seen some fascinating examples in my own use of ID tracking. Sometimes it's useful to dig into one ID just to see how unexpected the detailed data may be.
For example: one ID on one ecommerce site in one evening, placing an order:
A customer appears as 9 different sessions in GA.
She used 4 different devices - Android Tablet and Mac most of the time (the order was on the Mac) but also an iPad and a Windows machine.
On the Android Tablet she used two different browsers: Chrome and one built in to an app (I think)
On the Mac she used two browsers: Chrome and Firefox. The purchase was on Mac Chrome.
So that makes 6 different device/browser combinations.
She maybe used 4 different PPC ads to get to the site and also a lot of organic search.
So that makes 8 different device/browser/channel combinations (each of which = a new session in GA terms). The 9th session may have been because she closed the browser on Windows Chrome.
The real life session lasted from 20:39 until 22:21 although there was a 20 minute break right near the start. The purchase was completed at 21:25 after 51 page views -- call that 45 minutes.
After purchasing she looked at 50 more pages over the next hour.
Of the 9 sessions, 5 were reported as "new sessions" in GA and 4 were "returning".
What was going on? Was the site broken for some browser/device combinations.
When I say "she" was there just one person involved. This was in the evening. The site sells products for children. Was it two (or more!) people looking at various options before placing the order, all of them on devices which were kept signed in for convenience to the one account?
We just don't know. In fact this seems to be a classic example of the saying "them more we learn, the less we know" -- the more data we get and the more details we get, the less secure our old assumptions become. I haven't yet looked at the pages viewed during those hours. Of course that would give yet more illumination, especially about all that 'post purchase' activity. But who was actually behind all those cookies, and what were they thinking -- we're still guessing.