memtest86 doesn't seem to be testing all RAM memory

If your machine has multiple memory sticks, you need to test each memory stick alone.

According to your screenshot, your test is not complete, only proceed 17%, we need to ensure the pass at the bottom is 1 or more, this value indicates the number of test count.

About 2048M – 3327M, don’t worry this value, it only indicates what memory you have tested, look at the following value, 1279M of 8190M, you know, 1279=3327-2048 and 8190 is your entire RAM. The digit here doesn’t stand for your memory stick’s volume.

MemTest86 is reporting 8190MB to be installed. So all the available RAM will be tested.

However not all of the 8GB will be available. Some of the RAM will be used by hardware mapped into the address space. See memory mapped IO. Some addresses are also reserved by BIOS. See the e820 memory map. This also has the effect of fragmenting the address space.

Also you can't look at a memory address range in MemTest86 (nor Windwows) and know which RAM stick a particular address corresponds to. Interleaving with multiple memory channels and other factors make the mapping very complex.