Don't be shocked. K's who can count to 20 are most likely doing it by rote. Many of them have little understanding of the significance of each number, conservation of number, etc. Counting IS K curriculum so please please do not lable one preK better over the other. Hopefully kids go to developmentally appropriate preKs (and K and 1st....etc) where pressure is not put on them to learn K curriculum. Many are simply not ready. It's early in the school year. Concentrate on real understanding of NUMBERS. What they look like, counting, writing the numbers. Understanding 'oneness' and 'twoness' and 'threeness'...If you can get them to REAL understanding of numbers to 10 this fall you are in good shape. (and I mean REAL UNDERSTANDING!!!!)
Try graphing, counting many many many things, counting on, etc etc.