I don't want to rain on anyone's parade, but relative position on Amazon doesn't equal volume. Right now the PHB is 28th in the Teens section, and if you look at what's selling better than it (several individual editions of the same book, an expensive audiobook version of the Hobbit (!) and some titles that I'd charitably simply describe as obscure) you can't help but wonder if that position translates into hundreds or dozens of copies (I suspect the latter).
Again, everything is anedoctal but if you get the chance to investigate the actual volume of Amazon sales, there's people who's been on number 1 of all books with volumes that ended up with 4,000 copies in lifetime sales.
We're talking of really small numbers here.
I wouldn't bat an eye if #25ish on the Teens chart translated in 10 copies a week.
We need a better metric. This is completely immaterial.
While the above is true, it misses the point slightly.
Saying that a book got to #1 in books on Amazon means that it has definitely had one moment of good sales.
Saying that a book has hovered around the top 100 for two years means that it has consistently outperformed almost everything else on Amazon. The point is, cumulative sales are what gets you volume, not peak.
Almost every book that gets into the top 100 - and even almost every book that hits #1 - vanish from the top list very quickly. Books that stay in the top list have consistently high sales. Cumulatively, it is safe to say that the PHB has outsold almost every book that has been #1 since it was released. Does that give us absolute sales figures? Of course not. You can't use the ranking on amazon to make precise statements about sales figures. What you can use it for is to make statements such as "The 5e PHB is one of the most popular books that Amazon has sold in the last two years". With a bit of investigation, you could make statements such as "The 5e PHB has had such consistently high sales that, over the two years since it was released, it is almost certainly in the top 20 books on amazon for gross volume shipped" (20 seems reasonable based on very simple investigation; the real answer may easily be ±20 of that).
To put that more into perspective... It is almost certain that the sales for the PHB have been so consistently high that Amazon will be having to treat it differently to the vast majority of their stock; making more frequent orders and keeping more stock distributed in their supply chain. It's likely that this is handled by their automated tools - after all, Amazon retail is basically a massively automated supply chain and distribution network - but it's within the bounds of possibility that someone has had to check the automated system is doing the right thing (or fix it when it didn't) specifically for the 5e PHB.