I noticed that for some clients (in client letter), their 1st and 2nd federal estimated tax payments are lump in one amount in client letter.  but, others have separate amounts and both due 7/15/2020.

for example, one client show:  due date 7/15/20, federal $2,000 / 7/15/20, federal $2,000 / 9/15/20, federal $2,000 / 1/15/21, federal $2,000.

Others:  due date 7/15/20, federal $4,000 / 7/15/20, federal $0 / 9/15/20, federal $2,000 / 1/15/21, federal $2,000

Does anyone know why?