Alright, so, I cleaned up my disk. Finder shows 63 GB free. "Storage" tab in "About This Mac" shows the same...
However, using QuickLook on my disk (one and only disk I got with computer) shows 17 GB free only?!?
df -h
 also shows really strange output:
Filesystem      Size   Used  Avail Capacity iused               ifree %iused  Mounted on
/dev/disk1s1   113Gi   97Gi   13Gi    89% 1427126 9223372036853348681    0%   /
devfs          332Ki  332Ki    0Bi   100%    1150                   0  100%   /dev
/dev/disk1s4   113Gi  2.0Gi   13Gi    14%       2 9223372036854775805    0%   /private/var/vm
map -hosts       0Bi    0Bi    0Bi   100%       0                   0  100%   /net
map auto_home    0Bi    0Bi    0Bi   100%       0                   0  100%   /home
I am not using and never used Time Machine backups on this computer, there are no "purple bands" in "Storage" window, I tried to reindex the volume (by adding and removing a folder in Spotlight settings), my trash is completely empty, I tried to reboot couple of times...
Why is this happening??