The Series A crunch isn't caused by a decrease of Series A rounds, from various studies (from cbinsights and others) Series A investment is actually relatively stable.

What's changed is that you're getting far more companies who have raised money at the seed level. So the percentage of companies getting on follow-on investment at Series A is dropping while the absolute number isn't.

Also doing it over a 30 day period probably isn't long enough to take into account the vagaries of randomness (rounds may happen erratically, there's typically a delay before round gets added to crunchbase, etc.) - probably makes more sense to do it from start of year.