The number is showing the total number of database entries for the selected build, when it should be showing total number of database entries minus _setscene tasks. The number reported here should match the "All tasks" number reported by the tasks page filters.
merged to master http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?id=5345985b47d6716732304ed4f28494598a21c7ed
Verified on master: 2d1660112e54653f7bb763939d0416472c49fe01