The skew between the two legs of a differential pair causes differential signal to common mode signal conversion as well as reverse mode conversion. Mode conversion distorts the differential signal and thus creates a reduced differential signal. A common mode signal might re-convert back into a differential signal, creating increased differential noise. Some common sources of skew are asymmetrical structures from manufacturing variations or unintentional return path asymmetry, glass weave, and trace length mismatch. In Sources and Compensation of Skew in Single-Ended and Differential Interconnects [9], you will find a review of various sources of intra-pair skews.