This again an answer to my own post, sorry.
Indeed the RAW file format does not extend the range for the Canon 350D (which uses only 12 bits), so full white on the Canon ends up at 0.0625 in PixInsight.
For some unknown reason the file explorer does scale the data for the histogram.
I think it should at least be an option to scale when loading a CR2 file, to be consistent.
--bitli