I wanted to know what was the free available space in my internal HD (I actually have a SSD). In Finder, if I trust the Show Info information (right click or cmd+I on Macintosh HD icon in the Finder sidebar) I get a certain available space. Otherwise, if I just press the spacebar (quick insight), I get a different value, it seems that it's not updated. Finally, also Disk Utility reports a different value.
Why these three different values? (this gap first appeared after upgrading to El Capitan and trash is already empty).
Available space displayed when clicking
Available space displayed when pressing spacebar:
Available space displayed in Disk Utility: