milmo, Welcome to the forum.
3-4 BPM can be an allowable/expected difference between sessions, IMHO. True enough a new chain can become more flexable through use or may not initially mesh perfectly with 'used' gear teeth, creating a miniscule amount of added resistance.
Not too long ago I replaced a XTR chain with a SRAM chain, without precise measuring eqpt. my seat of the pants impression was that there was no increased drag.
Brad