Might be perhaps because Corona is more memory bandwidth taxing that most multithreaded applications.
2680v2 is still stronger in all other benchmarks than 2650v3 (3.05Ghz all-core turbo vs 2.6Ghz all-core turbo cannot be nullified by Haswell vs Ivy Bridge improvement, which isn't so drastic).
On other hand, Haswell-E is on DDR4, most builds usually at 2166mhz, while most Ivy-Bridge-E are 1600Mhz (my board for some example won't even accept to run at such clock, so I am forced to run at 1333).
This can make a difference if Corona can use it well.