It's the difference between the linear response of the digital sensor and the logarithmic response of human vision, which is taken into account by display devices such as monitors.
An 18% grey card "properly" exposed is only RGB (127,127,127) after full gamma correction has been applied to the raw data. When using "log" recording, gamma correction is reduced, thus it does not apply as steep a curve, and applying the rest is delayed until post production. In the "log" file (whether Sony, Canon, or Panasonic, etc. - they all do it similarly) a lower gamma value has been used and the recorded values aren't raised as much as they would be with an application of "full" gamma correction. It's up to the post processor to decide how to shape the final gamma curve as well as the "s-curve" at each end.
Note: When viewing a true "raw" still image file on a monitor, you're not actually viewing "THE raw file." You're looking at one of among near countless legitimate possible interpretations of the raw data with demosaicing, gamma correction, etc. applied to that data.