[foxfluff]

Are you saying your differential line is causing EMI if it's not length matched?

Or are you saying that the receiver won't do a good enough job cancelling common mode noise if you're not length matched? For that, I'd like to have a reference. Otherwise I'm not convinced this is more critical than the max skew to keep your crossings in the right region.

[lmilcin]

Differential pair minimises EMI by having constant common mode potential over the length of the line (and wires close to each other). Meaning if you look at the pair from afar the average of their voltage at any point in the line is constant. Or you might say that the noise from both lines cancels each other. From afar, it looks like there is no signal traveling the length of the line (to an approximation, because there is necessarily some distance between the wires meaning the cancelling is not perfect).

If the signals are shifted in time, though, that is no longer true. From afar you will see places in the line that have different common mode potential. Where one signal edge lags behind the other the average will no longer be the same as for the rest of the line. From afar this looks like high bandwidth signal travelling down the line.