Well, I finally found, after re-re-…-reading the online manual. I cannot find the section again at the moment, but it was something about local presets management vs global presets management. This was late before going to bed.
So today I had a look at Preferences / Presets / Exposition and noticed there were two entries checked : Default for scene related and +3 IL - the culprit !
I had to expand the window and scroll to the right to see the checks.
I still do not know how this can have happened, maybe some wrong typing at a given moment, but it was during my previous session just before reinstalling dt.
Much headache for just a little check mark. 