Source: native English speaker in Europe. I have to explain/reword several words/expressions per day to people who would be by all means considered fluent.
(all numbers in this comment were estimated based on experience)
Im also a native English speaker and have to explain English words daily to other native English speakers. Dont really think that matters. Some words are more common than others.
And to each their own. Fluency is a bad metric because it means something different to everyone. If you live in a language, work in a language, and have friends in a language, most people would consider that fluent. I've met many, many people who qualify with a much lower comprehension level than 90%.
Also, speaking from experience, I'll often "comprehend a sentence 100% in another language". Then I'll really listen to it again and realize I'm not really sure about half of the words. I have a vague idea of most of them and in context my brain get's it and self-reports full comprehension.
I think "full comprehension" is a substantially higher bar than "fluency".
I get it, and in my experience, when I find myself relistening and not being sure about "half the words," it means Im not fluent!