The PB version is automotive and "picopower". Also still with "preliminary" datasheets, so I think they have bad yields or too much demand.
Or some other problem that is soon to be added to the list of errata. Of which there already are a few.
Just noticing that now. The product page datasheet link says:
"(file size: 634KB, 24 pages, revision C, updated: 03/2015)"
But there was an update the next month:
Rev. 42176D – 04/2015
The Rev D errata for the 168PB says "no known errata", so maybe it's ready for volume production now.
p.s.
I hope they update the rest of their datasheets to use colored graphs. In some datasheets the black lines cross/overlap so it is impossible to tell which line applies to which label.