Because cases are generally processed in the order they were received, and as the late, great @geowrian always said, "USCIS is like a large grocery store. Some lines are just faster than others.". You are overthinking. Try to relax and remember that patience is a virtue in the world of immigration. Good luck. I think you are only a few weeks away, at most. The average Visa Journey reported case is taking almost 9 months at California.