Not entirely true. It depends what you mean by prior knowledge. Often there is 1 question that requires you to just recall a fact, plus 2 or 3 more that require some sort of background knowledge, although not necessarily specific. There's one practice paper that, for example, requires you to know that noon is the hottest time of day. Logical, but you won't find that answer in the passage.
What it sounds like you're talking about is some knowledge of how equations fit together. If two quantities are related such that as one increases, the other decreases, then we say they are inversely related, and their equation will look like a=1/b i.e. one is in the denominator.
For a fuller discussion of this, plus a pretty comprehensive review of the outside knowledge you need and some great strategies, look at my book: https://toptalkedbooks.com/amzn/1771365692
21 then 31! That's crazy! Yeah, you just need more practice, as it seems like you don't really understand the science properly yet. Skip the December test date and work towards the Feb date, or even April. If this is your third time already, don't write the test again until you're completely sure you've mastered the science section.
Your other scores are obviously higher, and the typical pattern is that students can bring their science scores up to where their reading scores are, because the skills are similar. But the science section is different, because while you're totally used to reading text, you're not used to reading mini scientific papers, which is what the science section is.
Hopefully it should end up that you can still do ok on passages where you don't understand what's going on. A lot of the time it doesn't matter for the questions. One of the best strategies to address this is to just grab a practice test, and answer the questions in whatever time it takes for you to figure out the right answer. Doesn't matter if that takes you 50 minutes. You gotta figure out how to walk before you can run on this test, if that makes sense.
Here's the short version of my advice for the science section: https://toptalkedbooks.com/amzn/1771365692
If you want a cheaper (and I think better) alternative, consider Understanding the ACT Scientific Reasoning. Here's what it has the "for the Love of ACT Science" doesn't have:
This is a new book, published this year, so it's not very well known yet, but if you read my Amazon reviews, you'll see that private tutors are now using the methods they've read in my book. I've been tutoring since 2014 and almost none of my students have science as their lowest score. It's almost $10 cheaper than "For the love of...", and I think it's a lot better. I don't waste space in the book constructing practice tests, because you can find them online for free and it's impossible to replicate the ACT's anyway (the book has links to my own website where you can find them). Instead, I guide you as to how to practice effectively.
Find it on Amazon here: https://toptalkedbooks.com/amzn/1771365692