So you're not talking about left and right channels reversed correct? More or less each driver zigs when it would otherwise zag? Apologies in advance for using the highly technical verbiage
Out of phase means that when the soundwave is supposed to be positive, it is instead negative. Which means, when the driver is supposed to be pushed out (on the speaker, or on the headphone) it’s pulled in instead. That happens on both channels, assuming, as they would be here, they are both out of phase. in reality, it is something most will never notice. Some (older amps used to have a button to select phase. The reason that is not a popular option, it is of little value, so if you do compare two Headphones simultaneously, and they are exactly the same type on the same amp, there is a tiny chance you might notice a small difference, and even then you are unlikely to here it or care about it.