Analytics and reporting

I use moodle 3.9.11 with training sessions 3.7.0 (build 2019112200). Students have permission to see their reports. 

Some users insist that there is a problem with reports. Not many, but enough to trouble me.
They say for example that although they had viewed a scrom activity for... 2 hours, the report shows only 1. 
In addition, a couple of them claimed that when they viewed their report after 2-3 days, time in some activities was not the same. It was less.

Αt first Ι thought they were wrong, but now I am thinking something is going wrong.
One student had almost 40hours in "Other course use time" and only a few hours in activities. I found it strange since student says that he was viewing scrom activities all this time.

I have done many tests but I haven't found any error.  what can go wrong?
is there a possibility that the hours are not being measured correctly? And apart from that, how can the hours be reduced for some students (2 so far)?

All these students attend different courses. I cannot find any pattern between them. Everything else works fine on the site.