I just did mine last week, and of course ended up doing it twice because I was off by a tooth on both cams my first attempt and it ran slightly better than awful.
It looks like the belt is putting increasing tension on the tensioner. That means that the slack that is supposed to be towards the tensioner and spread out evenly with a single rotation is being taken away from the tensioner, almost like the cams are at unequal rotational speeds which just does not seem possible.
I would leave the VC off, pull the spark plugs, and spin the crank by hand slowly to see how the tension is being distributed. I am very curious how this turns out. I am always impressed at how seemingly impossible things turn out to be something so simple.
You might actually need a new timing belt though once you figure it out. You aren't supposed to put any more stress on it than what the spring provides.