Haswell and Broadwell processors certainly allow the uncore frequency to be higher than the core frequency. If the BIOS has configured the processor to operate in the "energy efficient turbo" mode, then the hardware will typically push the uncore frequency to the maximum value and keep the cores at an intermediate frequency when running STREAM. The detailed behavior depends on lots of settings, not all of which are documented clearly...
On the Xeon E5-2660 v4, the maximum uncore frequency is 2.7 GHz, so if the core is pinned to 2.0GHz a ratio of 1.35 is reasonable.
In the uncore performance monitoring guides, "UCLK" stands for "Uncore CLocK cycles". Most of the boxes in the uncore run at the "uncore frequency", so UCLK counting is available in several places. The PCU, the QPI Link Layer, and the DDR use other clock frequencies, so they have their own clock counting events that can be used to determine relative frequencies.
