Is that average 6sec for one fight, or for like, 100 fights?
Because I'm pretty sure that sometimes you can have as low as 4s eCD on DS and I've seen logs like that.
Yesterday, in a pug I had 5.0s eCD and did 16656 eDPS, that was on a pretty long 3:53 fight. So, doesn't look like 4 minutes is enough to average it to 6 sec.
Chance of 2-piece proc is 40%, but in particular fight where you swinged 100 times (~4min), you may have 25 procs, you may have 55 procs. It happens. Prioritizing DS > CS, you just making sure to utilize tier bonus to it's fullest, isn't?