I agree the date seems random. Q, I would point out that the monthly allocation for EB1 and EB2-WW is much higher than that of any single country. However, I agree that EB2-M and EB2-P probably have a higher demand than 250 a month and it's hard to explain how they have the same cutoff date as WW.
The dates show CO will be super-cautious this year. Burned once (2007) and twice (2012). Don't think he would like to be brunt thrice.
USCIS seems to be adding 6K India, 1 K China and 2K Other cases per month.