In your video, your Scourge II recast actually starts at 6 second recast, rather than 5, so it's not a mixup of spell recast timers. I bet if you had used some other ability with a decent sized cooldown, it would be off by 4 seconds as well. I had this issue yesterday too (my Triangulate V recast and the duration for the buff it granted were both off by a few seconds), sometimes exiting the game (completely exiting, not just logging out) and logging back in fixes it and sometimes it doesn't. This has been an issue for quite a few people for quite some time now.
I would just like to clarify that this has nothing to do with Scourge II unless you can make a video that displays one ability using the proper cooldown, and another ability using the improper cooldown.