How many tasks running at the same time on that phone?
Is that phone used for something else?

When I tried on my main phone, I saw the same. Since I don't bother with Android and BOINC, I didn't really care but it seemed like the more tasks I ran (up to 8), the higher the elapsed time/CPU time ratio was.

That might explain the problem. If other apps are using the processor, then it is normal if the elapsed time is significantly higher than the CPU time.

Do you leave it on during the night? Can you tell any difference between tasks that run at night versus the tasks that run during the day?

So, a CPU or as Intel likes it to become a XPU, has limited resources for certain processes, 8 of this 4 of that, 2 of these functions. What you likely have is competition for these 'not all CPU parts have 8' resources. I let me phone only do 1 out of 2, tablet 3 of 4 and get better efficiency than my desktop... 99.8%. If multiple make the same observation on these 8 core ARM's then big/little has taken on an extreme shape. See https://www.youtube.com/watch?v=KClygZtp8mA#:~:text=64K subscribers-,ARM big.,maximum compute capacity when required.

My understanding is that even if the CPU percentage is set to 100%, when there's another app running, the CPU will be used for that app too at the same time, which slows down the process of BOINC. It is not obvious if you use a high-end phone with a very powerful CPU but if it is very obvious on an old phone or PC. I set the CPU percentage to 100% in BOINC on my old PC with a Pentium4 CPU. When there's no other software running, you can see the number in Progress ticks faster. But if I open another software, like Firefox with 3 to 4 tabs opened, the number just ticks slower. Most smart phones run some apps in the background, the more apps are run the slower BOINC is.

Another point is that even though there may be multiple cores in a CPU, it doesn't run a specific task with a specific core, like task1 uses core1, task2 uses core2, no it doesn't run that way. Instead, every task is getting through every core. I don't know about Android phone but at least this is how it's done on PC. Check the Task Manager and CPU Performance and you can see all cores are running on a single task. I have an old PC with dual PIII CPU. Without any setting, it's way too slow to run BOINC and surf the net at the same time (only 50% CPU uasge). But if I designated CPU0 to run BOINC and CPU1 to Firefox, still 50% CPU uasge, problem solved, I can surf the net much faster without compromising the 100% CPU0 on BOINC. I don't think there's any difference on an Android phone. The only problem is there's no way to set specific task to specific core on an Android phone. I don't know there's any app or any setting to do that.