From the wiki: Another problem for this list is the method of determining the mass. Such methods, such as broad emission-line reverberation mapping (BLRM), Doppler measurements, velocity dispersion, and the aforementioned M–sigma relation are not yet well established. Most of the time, the masses derived from the given methods, contradict each other's values.

So what that means is "our best guess of a list of most massive black holes".