So this might not be an issue per say. Each bud has a separate battery which produces separate sounds (L+R earphones have different audio tracks for stereo sound, therefore one could require more power than the other).
Also, one bud acts as the 'master' bud which connects to the phone, and one acts as the 'slave' which connects to the master. This is why on external devices, the buds show as one device. Whichever bud is assigned as "master" will use more power as it has to connect to the phone and the other bud, where as the slave bud only has to connect to the master, if that makes sense to you.
Hope this bit of info helps.