Is Nautilus reporting wrong free space?
Solution 1:
By the looks of it, Nautilus is reporting GiB ( = 1 073 741 824 bytes) and PCMANFM is reporting GB ( = 10^9 bytes).
In other words:
81.3 GiB = 81.3 * 1 073 741 824 bytes = 87.2952103 × 10^9 bytes = 87.2 GB