Why does the myPhonak app show a difference in battery life between the left and right hearing aid?

A difference up to 30% in battery life is considered to be within the tolerance range.

There are a few reasons why there can be a difference in the battery consumption.

The phone ear - the hearing aid side that is connected to the phone for streaming media is considered to be the phone ear. Connectivity of the phone to the phone ear hearing aid consumes the battery. The contralateral hearing aid receives the streamed signal from the phone ear so less power is consumed.
Hearing aid features like binaural voice stream technology (BVST) connects to one hearing aid (your phone ear) and then send information to the contralateral ear. If you are in situations where BVST is used, such as in noisy situations, you will see a difference in power consumption between the 2 hearing aids.
The hearing loss – if you have asymmetric hearing loss on both ears and using different receivers, this can cause a drain in power.

If you are concerned about the drain in battery consumption and it is not providing a full day of hearing, please consult your hearing care professional.