After doing exhale breathholds (about 1:00 - 1:30) with around 2 min break in between, I noticed that they get easier each round, but the value of my finger oximeter drops significantly more (e.g. value is 10% lower on the second breath hold ect.). I was wondering why this is the case, I try to breath normally during the break (i.e. not hyperventilating). I thought the body should adapt and consume oxygen more slowly, because of the diving reflex or something after repeated holds.