..but it seems to me that it underexpose one half to one stop, don't you think?
Yes, a number of people including me have observed this behaviour. Due to the fact that X3F files have a lot of room for highlight recovery, I normally overexpose by 0.7, thereby exposing very much to the right, and thus minimising shadow noise. Of course these requires using X3F raw files. But this should be standard procedure anyway.
An example for highlight recovery using the DP1M (which has the same exposure characteristic) can be seen here:
The clouds on the left of the skyscraper have been recovered in Sigma Photo Pro by an overall compensation of -1.0, and a X3F fill light of +0.5.
Robert F. Tobler