I wonder if it is a dual-MOBI.
Install the KindleUnpack plugin for calibre, see if it produces a MOBI as well as an AZW3.
I believe calibre will simply treat dual-MOBI as MOBI.
...
Rest assured though, that it is still the same book.
When the publisher uploads it, the same source document is converted into a high-quality AZW3 for newer devices, and a legacy MOBI for legacy devices that don't support AZW3.
Some old books are only available in MOBI, since they predate the existence of AZW3 and were never updated.
I prefer adding books to calibre directly from my Kindle.