ive noticed some inconsistency in volume as well. normal handset volume sometimes feels a little to quiet despite being up to 100%. speaker volume is decent though i noticed theres a separate setting for it i think in the settings that shows up only when your in that mode. i had to do this for bluetooth which i couldnt hear anyone at all until i went into the settings while on the call and the volume was different from the standard call volume setting when not on bluetooth.
maybe this is causing some inconsistencies in user reports of volumes. i had been finding it confusing having to have all my calls at 100% volume all the time