When I was analyzing a tree of an NTFS volume, I have noticed that directory sizes do not match the sum of the file sizes that constitute them.

For example, there are four files in a directory: one of size of 1.0MB, one of 768kB and two 2.2kB files.
QDirStat 1.5 shows me that this directory contains 902.3 kB of data which is roughly half of what it actually contains.

This bug (?) does not seem to appear on EXT4 file systems.