Great observation!
Cannabis seed size can vary from strain to strain, and within the same strain depending on each seed's maturity.
I use a sieve that has five layers stacked on top of one another. The best seeds usually fall within 2 layers... 1,2 or 3,4 or 2,3 and so on...
Everything above is usually deformed and won't fit through the biggest holes because it's misshapen.
Everything below is underdeveloped and doesn't meet standards I've set to reduce customer issues.
What's left is two distinct sizes, both comparable in quality.
"Why do you call the top of two layers A seeds and the bottom of two layers B seeds?" I am often asked.
It's solves two problems.
1) When packing up seeds, the switch to B seeds sets off alarms and reminders to pollinate seeds that will soon be sold out.
2) I got tired of answering "there is nothing wrong with the smaller seeds in your pack" & "Yes, you may have a replacement" to customer emails regarding the varying size of seeds in each pack.
Finally you have a pack of seeds that are uniform in size shape and color.
Quality control.