Why OS shows a different available space than the Disk utility? What should I do?

Recovering the free space when deleting from an SSD is not instantaneous. The drive must run its garbage collection routine to reset the old storage so it can be used again.

I don't know how long that takes, but I don't imagine it should take more than overnight.

And, the Storage Displays are always wrong (at least you should assume that), so make no decisions based on them.

They key difference isn't the number, but the description. Disk Utility is telling you how much "free" space you have. The Storage display is showing how much "available" space you have. They are not the same. Free space is unused space. Available space is free space plus any storage that could be freed if necessary.

alejandro67 wrote:

Well... The problem is that I needed that space to install a program but the installer sees it as taken. That was my problem.

I realize that. Unfortunately, I simply don’t have a good answer for you. If you wait 24 hours or so, the space may finally be released as local snapshots are deleted. You can force the issue by running the following command in the Terminal:

/usr/bin/tmutil thinlocalsnapshots / 10000000000 4