How does Google highlight meta descriptions?

How does Google choose what words to highlight in the meta description? My meta-description has keywords my competitors have as well - yet Google chooses to highlight their keywords, but mine remain regular? Attached an example

I googled "test case" and navigated to page 2

Some websites have the word "test" highlighted, while others don't. Specifically, result 5 has the word "test" highlighted while result 2 doesn't. Anyone know how these keyword highlights work? If your words aren't highlighted in the meta-tags, what you should do or what you're doing wrong?

It's just you need to have that keyword into your meta description. Doesn't matter whether you are on first page or second page google will highlight that keyword in the description. Make sure have exact match keyword mentioned in the description.

Because test is only a partial match. The others have test case highlighted. Where google could match the full keyword it did, when it couldn't it matched partial.

I doubt getting the word highlighted would have any significance, and furthermore, the meta-description is fully automatic and in some circumstances, it is dynamic in nature, meaning google would pick their own meta-description, rather than following the one that you have keyed in for your article.
In so many circumstances, I have keyed in a nice description, but somehow google would pick something other than what was written for the meta-description.
Getting ranked higher - at the 1, 2 or 3 spot would be much better and more significant as compared to getting the words highlighted - these are the true traffic spots, and the best conversion ratio spots for the SERP.