Moodle 2.6.
I have just had a case where a teacher had set up 5 columns in the gradebook for the marks his students got during some off-line lab work. As he was only interested in the average mark for the 5 pieces of lab work he had created a sixth column and populated it using the average function on the other 5 columns. From his view of the gradebook everything looked fine and the students had the correct mark, however a student reported that he had 44483% for the lab work. I logged in as several students and they all had this very same mark. To sort the problem I revealed the other 5 columns to the student and hey presto, the calculation worked correctly.
Has anyone else come across this and is it a bug or a feature of Moodle as I could see that sometimes you may wish to use values from hidden fields in a final calculated column.