My mobile 6 months old battery 91% why

Referring to the "Your battery's maximum capacity" section of the support article: iPhone Battery and Performance - Apple Support

"A normal battery is designed to retain up to 80% of its original capacity at 500 complete charge cycles when operating under normal conditions." Even rechargeable batteries get worn down and 1-2% health loss per month is perfectly normal. According to iPhone Battery and Performance - Apple Support , Apple apparently only considers it unusual (and warrant-able) if a battery drops below 80% maximum capacity in less than a year after you receive it. Ergo, some storage loss as you use it is anticipated and acceptable. Prorating that possible 20% drop over the first 12 months equates to 1-2% drop per month being within the range of what Apple finds acceptable. "The one-year warranty includes service coverage for a defective battery. If it is out of warranty, Apple offers battery service for a charge." In other words, if maximum capacity is below 80%, the battery is below normal performance and worth replacing. A notification will appear about this. You can still use a device with a lower health percentage than 80% but for optimal usage you may wish to consider having it changed when it reaches 80% battery health.

- If battery health drops a few percentage per month that is normal.

- If battery health drops below 80% before the end of the first year, contact Apple about a warranty replacement. (Apple will test it to determine its actual eligibility.)

- If battery health drops below 80% after the first year, consider getting the battery replaced but anticipate paying for it.

Is the phone new, meaning it was bought directly from Apple 6 months ago, or is it a previously used phone that is new to you? If you enter the serial number here Apple - Support - Check Your Service and Support Coverage it will tell you indirectly when the phone was first purchased, as the warranty expires one year from the purchase date.

No guarantees, but try using Wi-Fi as much as possible, since cellular in general tends to use up more battery power than Wi-Fi.