David, thanks for your answer.
The correct Moodle version is 2.7.1 (sorry).
...in the meantime, I run some other tests (on demo.moodle.net), where I verified almost the same behavior and possibly found an answer (sort of).
While I was setting up the activity, I switched briefly to the closing phase and took some screenshots for tutorial purposes. That, as you might put it, "pushed the grades into the gradebook". Ok, that could explain why students got a grade, still...
Problems (on both versions):
- Grades for submission are assigned for all submitted work, no matter if
they were graded (by peers) or not at all (in the latter case, NULL is
converted to "zero"). I figure the rationale here is: "OK, they submitted something, no one assessed them, if teachers do not override the grade Moodle should assign a grade, so their activity is marked complete, at least". Makes sense... still, teachers could use some sort of warning for submission that were not graded at all by peers
- Assigned grades do not get blanked (i.e. back to NULL) once the activity is switched back to submission or even setup phase. (OK, so I will not go back and forth through the phases. Ever. )
On my version:
- All of the students got grades for submission (i.e. NULL converted to "zero"), even if they had not submitted anything at all.
Note: on both versions, no one got any grade for assessment (i.e. NULL stayed NULL), and that's OK, because no one assessed anything.