This is actually due to OSX and it's purgeable files system.

OSX reports to users that any space that is filled with puregable files (system backups from Apple time machine) is free space.

Unfortunately when an installer asks about free space OSX does not report purgeable files as free space (yep Apple programmers didn't think this through).

So this creates the scenario that the user thinks they have lots of free space but installers see no free space at all.

The fix is to just clear down the purgeable files so the disk space is now actually free and reported as free by OSX to an installer and now everything will work fine.