Trying to divine the quality of a school from the "trend" in their shifting ranking over time is a silly waste of time and brain energy.
Agreed. For example. one program a few years ago vaulted up like 5 places due to a large NIH grant. That grant trend was not repeated and they slowly drifted back to where they started. So what does that tell premeds about the school? Nothing. You have to realize that for the rankings to mean something, they have to focus on something that matters to you as a premed. The amount of NIH grant funding probably won't affect your life as a med student there. You need the place to have some amount of research if you are research inclined, but the necessary level probably doesn't even mean top 50. Bear in mind that a place with prominent people someone far away on campus (if even in the same state) with big research grants are reflected in the research rankings as much as the dude with big grants who teaches daily at the med school and has 20 med students working in his lab. So this is a meaningless stat. And yet it's the driving force of the research rankings. Primary care is worse. The driving force there is the percentage of folks who go into primary care. Which would be good except that primary care fields are generally non-competitive, so you cannot tell from this statistic whether this is a good place to go to for primary care or a bad place to go to for competitive specialties. Again a useless rank for you.
So what do you do? In truth there is some relation between the prestigious schools with prestigious faculty and the amount of grant money they bring in. This sometimes means better med school infrastructure as well. So the Harvards and Hopkins are perennially toward the top of the rankings list because they are prestigious and do lots of research. If prestige is important to you, then the top 10-20 on the rankings list probably factor higher in your decision process, whether justified or not. Will the programs ranked 11-20 be better than those ranked 21-30, 31-40? Probably not. You will get a great education at any of them, and you future will not be affected by which one you go to, nor will your board score change, etc. Now if you go to the lower end of the rankings, you might have to work harder to find a research job if you plan to research, might have to do more "away rotations" during fourth year if you plan to go into something more rare like rad onc, and might not have as well known faculty on your LORs (although anecdotally, those you do get might do more to help you). So there may be differences at the extremes. But don't kid yourself that the program ranked #5 is better than the one ranked #9, or that the one ranked #25 is better than the one ranked #35. You have to use these rankings with much broader strokes, if at all. Programs within 10-15 of each other are likely indistinguishable in terms of opportunities for things like research. And there is no ranking basis for determining "quality". So I wouldn't be married to rankings so much.
The bottom line is that there are relatively few med schools as compared to the number of people who want to go to med school. Half of all applicants don't get in, and these applicants are a self selecting group who have reasonable grades and scores for med school. So it's an elite bunch of students applying, and EVERY US allo med school as a result is quite good, quite competitive. Now if you think some outside company, looking at objective data like NIH grants can parse through this group of already quite good programs and say, this one is better because some brain lab in the next town got $100k more in funding, is going to give you good analysis of which is "better" then you probably need the help of a good brain lab.