TestLink

I have added EXEC_TIME and ESTIMATED_EXEC_TIME custom field to a project. To me, it looks like the calculation of execution time is not right or I don't get the logic. Here is the scenario:
1) I have executed 6 test cases and set the execution time 20 min for each. So the execution time in "Test Report" shows as 2h. This is OK.
2) I've created another build (e.g. V2.00) and started testing. I executed first test and set the execution time to 15 min.
3) I've checked the "Test Report" an noticed the execution time is 1.83.

What I don't understand is that why in second build when I execute only 1 test case and expect to see the execution time only for that test case (e.g. 15m), the execution is 1.83h. Here what testlink shows during executing in 2ndbuild, while the exe time is set 15m for each TC:

After executing 1st test case in 2nd build: it shows 1.83
After executing 2st test case in 2nd build, It shows: 1.75
After executing 3st test case in 2nd build, It shows: 1.67
After executing 4st test case in 2nd build, It shows: 1.58
After executing 5st test case in 2nd build, It shows: 1.5
After executing 6st test case in 2nd build, It shows: 1.42