I don't know what Bell curves etc are. But, I may understand it more clearly now...I was standing on tennis court practicing serve and stopped to think clearly (?):
If there is a 20% chance of a M6 each day:
for 1000 quakes.
for 200 quakes you have a quake on day 1.
Then you only have 800 quakes to check for day 2: you would have 160.
For how many have a 48 to 72 hour lag you onlu have 640...and 20% of that is...

So, OK, it should fall off if random. But, if it falls off faster than it should, then there might be a relation. Your numbers look like there might be excess quakes for 0 to 24 hours but I can't really tell.
Chris