Your headphones could be slightly damaged and have a minor imbalance only on certain frequencies. When you listen to songs that do not use those frequencies often, everything seems fine. You'll only notice the imbalance when listening to songs with voices/instruments hitting those imbalanced frequencies.

Test this by listening to a sine sweep on youtube. If the sound is perfectly centered the entire time then your headphones and your ears are fine. If the sound sways between left and right at certain frequencies then you found your problem.