iPhone 15 Battery Health dropping so fast

On 'average' for every 50 charging cycles, battery health will decline by about 1%. Battery health decline is not linear, meaning it can remain steady for a while, then drop a few points more rapidly. 268 divided by = 5.36. So your battery health after 268 cycles would logically be in the 95 to 94% range. Your battery could actually be at 94.9%, but the level is only shown in whole, not partial numbers. Thus, your phone is where it should logically be and there isn't anything wrong.

If you use a Charge Limit and prevent the charging to 100%, then the battery will not be able to calibrate for accurate state of charge estimates and you will see a larger drop once it does have a chance to charge to 100%. Even when using a Charge Limit, the phone will occasionally charge to 100% for this calibration, but not if you manually take it off the charger before that time.

You have nothing to worry about and your battery is exactly where it is expected to be.