Paul, it is probably an illusion created by room acoustics and imbalances canceling each other.
My system has always run 3 dB hotter on the Lt in my current room. There is an alcove on the left side wall. In preparation for a new turntable I finally completed the built in cabinet for that alcove. My left channel now runs 1 dB hotter. Now if my right channel electronics had 0.5 dB more gain switching channels like you did would cancel out the imbalance. This is what a balance control is for. Unfortunately, that imbalance is hardly ever equal across the audio spectrum so your frequency response curve is being shifted. This is where room control is very effective. For reference a 0.5 dB imbalance is just noticeable. A 1 dB imbalance is obvious.
My system has always run 3 dB hotter on the Lt in my current room. There is an alcove on the left side wall. In preparation for a new turntable I finally completed the built in cabinet for that alcove. My left channel now runs 1 dB hotter. Now if my right channel electronics had 0.5 dB more gain switching channels like you did would cancel out the imbalance. This is what a balance control is for. Unfortunately, that imbalance is hardly ever equal across the audio spectrum so your frequency response curve is being shifted. This is where room control is very effective. For reference a 0.5 dB imbalance is just noticeable. A 1 dB imbalance is obvious.