I get the following information in my timing report for an intra-clock path in the design:
The conservative common code of the clock path is the clock root of BUFGCE net.
So the clock skew = Distribution(Destination) 1.369 ns - Distribution(Source) 2.045 ns = -0.676 ns
The current reported clock path Skew is -0.679 ns < -0.676 ns.
Why does this discrepancy occur?
In some cases the reported routing and distribution delays of clock net are inaccurate in Vivado 2015.3, but the slack the timing path is not affected.
In Vivado 2016.1, the issue is fixed and the correct report is similar to the following:
In this report, the clock skew calculated from the Clock Net distribution delays matches the Clock Path Skew reported: 1.398-2.077 = -0.679.