SE told us some incorrect information recently when they tried to address the skill>acc and skill>att and magic skill>acc conversion rates.
There were multiple errors in the original data they provided us too, most of which got corrected.
I didn't say it's necessarily a matter of them unawaringly giving us wrong values for Threnodies, I don't think that's the case.
It's more a matter of elemental resistance - stat working differently from magic evasion- and that's quite clear to anyone who bothered testing Threnodies vs Languor and Frazzle.
Most of us assumed that -50 Thunder to be the same as -50 magic eva, but working only on thunder related stuff.
Quite clearly this is not the case.
So either there's something more at work there and we can't convert ele res to meva at a 1:1 rate (which is the most accepted hypothesys) OR the rate is actually really 1:1, but SE simply gave us wrong data for Thenodies.
I consider the second hypothesys less likely, but who knows?
What's for certain is that it's not 1:1 and Threnodies are much weaker than Languor and Frazzle despite affecting a single element.
Source: Sechs and all the people who participated in the three discussions concerning the issue on BG, FFXIAH and the official forums. I'm pretty sure you're skilled enough to use Google and find this stuff on your own if you really deem it particularly important.
If you ever tested what we're talking about yourself I'm sure you wouldn't need to, so the question is: have you?