Why is Task Manager Missing Tabs, and Not Jibing with Activity Monitor?

When I look at the Task Manager window, I don’t see all the tabs listed, that I have open. Also, some are listed twice, yet are just one tab, and the amount of memory in use, differs greatly from what I see in my Mac’s Activity Monitor.
The Task Manager shows the “GPU Process” at 1.3 GB, while the Activity Monitor shows it at 3.7 GB, a huge difference! Also, it shows Brave using a total of about 10 GBs of Memory! Although I see now, the Task Manager shows about the same amount of memory used… However, the TM doesn’t show as many open tabs, and I know earlier today, I was trying to find a couple, and they weren’t on the list, yet they were open in Brave, I just had to find them manually, instead of via the TM.

So, why is it using so much RAM, and why aren’t all tabs being shown in the TM?

I’m using the newest version of Brave, Mac OS 10.13.6.