You've run into the notorious Yosemite Core Storage issue. Core Storage is an Apple technology, first appearing in Lion, that introduces a new "logical volume format" layer between the partition scheme and the HFS+ filesystem. Its purpose seems to be to allow multiple physical volumes act as a single logical volume, as in Fusion Drive, and to facilitate whole-disk encryption.
When Yosemite was introduced, there were numerous reports from those running Snow Leopard noting a variety of strange partition-related issues, of which yours is a prime example. (See for instance
Yosemite: Questions; several posts, mostly in the middle third of the page, show folks grappling with such problems.)
I don't have time to research the subject for you, but you can start with John Siracusa's
Lion review, wherein Core Storage is outlined in depth, as well as his
Yosemite review covering the new 10.10-specific implementation. The latter might shed some light on the difference between your internal and external volumes.
It appears that reversion from Core Storage back to plain HFS+ is possible in some cases...maybe yours:
OS X Yosemite, Core Storage, and partition woes.Apple's documentation is characteristically minimal. The best (though terse) information I could find from the horse's mouth is in Yosemite's
diskutil man pages. Scroll down—way,way down—to
corestorage.
