Anyway, to your question which is a good one!
The kind of PCR based test used to diagnose COVID 19 uses what are called primer sets.
These are cocktails of short nucleic acid sequences which stick to specific regions of the RNA of virus in question. SARS CoV-2 has a huuuuuge genome, so there are lots of places to chose from to match.
If the right combinations of these primers stick to a strand of RNA, enzymes in the testing solution replicate it.
After the test is complete, researchers can see if anything has been replicated, if there is loads of stuff that indicated that coronavirus RNA must be in there. If there is none, then no coronavirus RNA is there, because the primers couldn’t bind and so nothing could be replicated. (With some room for errors).
Vitally, you need more than one primer in the set to bind for amplification to happen. Part of this is to prevent false positives - if one of the primers shares a sequence with something that isn’t the target for example. Lots of DNA sequences are shared across the natural world - which is where we get to the chromosome 8 point.
The code which makes up DNA and RNA is incredibly complex, but it is made up in each case of only four very simple and related components.
We call these A,C,T and G (or A,U,C,G in RNA)
Genetic code is just that - a code made up of these four letters, and there are a lot of ways to combine them, but because there are so few letters it is very common to see sequences repeated in all kinds of places.
We share short sequences of our DNA with viruses, plants, bacteria, flies, sponges, chimpanzees, you name it.
Sometimes this is because we are related to these creatures (like chimpanzees) other times it is just chance (like with SARS CoV2). Or it could sometimes be a combination of both!!
Sharing bits of code like this is incredibly common, and not at all surprising.
It so happens that one of the primer sequences used in one of the covid primer sets (there are many options) matches a short sequence from the human chromosome 8 as well as part of the SARS-CoV-2 genome. But this doesn’t cause a problem, because as I said above, primer sets work in groups.
Alone the primer which matches the chromosome 8 sequence will do nothing - it has to bind at specific points ALONG WITH other primer/s on the same piece of RNA to work.
None of the other primers in the set share their sequence with that chromosome.
Imagine it like a lock with multiple keys.
In that way we can make sure that only the correct target (sars COV-2) get amplified, and not get noise from human cells or bacterial cells or other contaminants (bits of spinach in the saliva!?!?) which share sequences.
That is why we do not get 100% of tests coming back positive. If it weren’t for sets then all samples containing human DNA (which they all do) would come back positive - which we know they don’t.
Primer sets are chosen based on their reliability, their ease of production and their specificity. This is tested extensively. Many primer sets exist and they are constantly developed.
If a set containing a sequence shared with chromosome 8 is performing well, then there is no reason to change it.
I hope that helped. 🙂