Recently me and another frost DK in my guild had been noticing rather low uptime on Unholy Strength (among other facets of RNG) and during a raid tonight he had decided to put his weapon with Fallen Crusader in his mainhand, this being for the last attempt of the night,
He commented that he had a much higher uptime on fallen crusader for that attempt and in fact almost had twice as many procs as I had
My buffs cast
The other Dk's buffs cast
For that attempt I had ran out of melee range to res somebody who had fallen off the ledge (who somebody ressed before I managed to) so my uptime was somewhat lower on top of me having terrible luck with RE and Rime but probably not to a degree to put me so much lower on FC uptime.
Because I was curious as to this and those parses weren't the most reliable due to other mishaps I did a quick test on training dummies only doing melee hits from plague strike and frost strike, 100 casts each, while I'm aware this isn't the most accurate test, it's a start and it was all I had time for.
While these results are obviously in favour of mainhand FC having a higher proc rate, I'll need to do further tests.
Mostly posting this here to see if anybody has ideas as to what may cause this besides sheer luck and to see if others would be willing to test for a wider sample.