I can't speak for everyone, but I am a bit of a non-traditional applicant, with my B.S being in Marine Sciences, and the Master's I'm finishing now in Earth & Environmental Science, so there were a few pre-reqs that I don't have completed right now. I applied to somewhere in the ballpark of 10 schools, with 6-7 needing maybe 2-3 more classes and the others needing maybe 4 more that I had not taken. I intended to complete the 2-3 prereqs required by all institutions, but for the remaining few I would gauge their interest in me as a candidate and proceed accordingly.
This strategy has saved me some money and stress, because working full time and doing a master's full time I don't have a ton of extra time for additional classes. However, hedging my bets in this way has put me in a bit of a precarious situation where I have to find time in my schedule to fit a 5th class for this semester.
Overall though, my real goal for this cycle was to get an interview or two, and I have been fortunate enough to get 4 so far and one acceptance! (Coupled with a few rejections and pendings lol)
I would say there are a good amount of people like me, who are relatively strong applicants who go into the cycle understanding the competitiveness and maybe have a few prereqs left for whatever reason, but still want to see who shows interest. That way if a second cycle is necessary you have feedback to work off of.