That's rather odd. Why not the more recent polls to begin with? This suggests a candidate could have entered really early, got a jump on some poll numbers but have fallen off the radar now but would still be eligible for the debate. In fact, that could push out a candidate that is currently doing better.
Candidate A: Qualifies
Candidate B: Doesn't qualify.
Well, that's why I explicitly said it, that not the most recent are used. Strange, but it is what the DNC decided. And it get's more relevant by the minute, because... tada!!!
Steve Bullock is running and apparently instantly qualifies via the poll criteria, as many past polls already included him. That means 19 have already qualified.
Bennet, Moulton, Messam and Gravel are currently trying to qualify. You have Bill de Blasio with an announcement of an announcement. And I think Stacey Abrams would be able to qualify, if she decides to run. That means six possible candidates might or might not qualify in the future. One more is no problem. With two more qualifying candidates Marianne Williamson is currently out. And also at this point is isn't enough anymore to qualify via donors alone. The third candidate who qualifies now triggers above rules. Currently Kirsten Gillibrand, John Delaney, Jay Inslee, Tim Ryan and Eric Swalwell look like the ones that could be endangered - if they not also meet the donor-requirement and therefore reach safety (for now at least).
I really thought it is too late for so many candidates to still qualify, and I thought 20 seats in the debate would be enough. But now it seems very possible, only one open spot left.