Ok. Possibilities are limited.

#1 - You added less salt than you thought, because they weren't 40# bags OR were diluted with something.
#2 - Your 'guess strips' were off . . . and so were the pool stores' strips.
#3 - Your pool has more gallons than you thought.
#4 - You're leaking and replacing a LOT of water.

I can't think of any others. If you add 160# of salt to 10,000 gallons of water, you are going to increase salinity by nearly 2000 ppm, every day of the year, everywhere in the world.