Search randomly fails, can't figure out a pattern. (nop 3.0)

I have several keywords in meta keywords fields of SEO

for now, lets call them

worda wordb wordc wordd worde wordf wordg wordh

these are the following things I've tried

worda works

worda wordb doesn't work

wordb doesn't work on its own, either

neither wordc or wordd work

worde and wordf work either on their own or when combined

wordg doesn't work

wordh works

I notice that for the most part, the keywords only work if they're also present within name or descriptions of the product.

worda is in full description
worde and wordf are in the name and short description
wordh is in the name and short description
the ones that don't work are not in the names or descriptions except for wordb. wordb is also in the full description and it fails to search.

Can someone explain how the search works a little better to me?

I added a new word that is not in the name, or either description.  it is only in the meta keywords and I can search for it.

I really need some help with this.  I tried adding some more words, putting words in quotation marks, then searching for them with and without quotations and nothing is working for some words, but others are working, and I can't understand why.

I understand that meta keywords are kind of outdated in the sense that search engines do not pay much/any attention to them. If you are not using the product tags option try coping your keywords there and include them in your searches.
https://www.nopcommerce.com/boards/t/14363/search-engine-could-search-in-product-tags.aspx