I had a similar or same issue, about mac(apple logo top left corner in menu bar) displaying approximately 50GB of free space, and disk utility something around 3GB. What solved it for me was to remove local backups that accumulated for time machine.
Run following commands in the terminal, which will lists local time machine backups(the /Data occupied the most space):
tmutil listlocalsnapshots /
tmutil listlocalsnapshots /System/Volumes/Data
you will see something like this:
com.apple.TimeMachine.2019-07-01-093318.local
com.apple.TimeMachine.2019-07-01-103122.local
...
and then, take date from the backup and pass it to the `tmutil deletelocalsnapshots` command to remove it, but before, make sure that you have a backup!!!
tmutil deletelocalsnapshots 2019-07-01-103122
My free space is back to normal and I am able to update.