Battery is not good enough (not like the manual says)

I have a 5% drain each 12 hours, 10% each day. Always in airplane mode, with OX, Wifi, bluetooth, .... OFF, only with HR ON and watchface by default. Should be 14 days in smartwatch mode.

Any advices? The watch says "20days battery left" when the battery is 100% charged, but it's really 10 days...It has no sense.

Thank you.