Why is there a discrepancy between free space and available space in my disk utility?

When checking my available disk space, the disk utility reports that my available disk space is 259 GIB, but my free space is listed as 55GIB. When i check the storage report for my HD i have plenty of space available. When i check my disk utility my free space of 55GIB's appears to be critical.

Why the discrepancy?

Which one should i believe?

How can i configure my disk to give me optimum performance as well as resolve the discrepancy from the two reports?