Please tell me why my iPhone battery health maximum capacity down fall 100 to 99,I want to know why?

Because Li-Ion batteries do not last forever. They will gradually lose their ability to hold a charge with time and use.

Don't worry about it. It's normal.

Because it is a consumable item. Every battery in the world, will have a reduced capacity over time.

Check the battery health again in 6 months. If it severely depleted, then start worrying.