My MBA has been behaving oddly recently. After startup, it initially displayed 321 GB as the available space, then when all the bits & bobs had loaded, it showed 369 GB free, this being the correct figure.
I booted it from an external SSD (Yosemite) and used Techtool Pro 9 (lacks support for Sierra) to create a Mac startup environment with seven apps plus TTP9 on a USB stick drive.
I then booted from the TechTool drive and played around (NO TESTS WERE RUNwith it a bit before restarting normally. Now after startup it just shows free space as 369 GB.