When you use this song, unless you're only summoning 1 rat I think that it summons 1 less on the initial singing, i.e. it summons 1 when it is supposed to summon 2, and summons 2 when its supposed to summon 3. What seems to be happening is that 1 rat is being unsummoned for whatever reason. this only occurs on the first instance of bard song and rat summons and not subsequent ones.