[xubuntu] Different file managers detects different free space sizes?

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http://ubuntuforums.org – I've only been using Xubuntu for about a month now, and I've noticed from discrepancies between Thunar and PCManFM, Thunar records my current free space as 10.8GB, while PCManFM as 11.6GB. The reason why I have two file managers is that I had to install PCManFM to change some file and folder permissions, PCManFM is easier to use than Nautilus. Why is Thunar and PCManFM detecting different free spaces? (Hardware)