Why did my iPhone 15 battery health decreased by 1 % in just 26 days ?

I’m not sure what you mean; is it that the battery capacity has dropped from 100% to 99% after 25 charge cycles? If so, that is exactly according to spec. The battery is rated to stay at or above 80% for 500 charge cycles. Doing some basic grade-school math that translates to a 1% loss of capacity for each 25 full charge cycles.

And actually, it may not have dropped 1%. If it dropped just 0.1% it would still read 99%, because the gauge throws away the fractional part of the capacity measurement.

In other words, your iPhone battery is performing at or above its specification.

That’s completely normal. Batteries will sometimes drop 2% in a month, then the next month drop 1, then drop 3 in another. It depends on how much you’ve charged the device. Dropping to 99% in 26 days is typical for the battery.

More info:

iPhone Battery and Performance - Apple Support