I have observed this for quite some time and done some looking into it. My sepcific theory is that the tank with lower enmity earns a bonus based on the disparity between their threat and the lead tanks, thus the closer the two enmity points the less that bonus (potentially down to no bonus at all).
I think turn 2 is a poor example because hate is never very heavily separated owing to the constnat switching. More over, turn 2 would just seem to confirm my statements about it apparently being based on the threat difference. You are always close to them on turn 2, so it should recive little/no bonus. Unfortunately, this example does not offer evidence either way to my specific theory. (Namely that when one tank's threat is substantially different than the others, a bonus seems to be applied to the lower tank based on the % of difference).
For reference, my cotank and I are very close in gear, both with +1 curtana. We are also both the same race. Regardless of which one of us starts with the hate lead, the other is able to chase them down, more it can be done with both of us in shield oath, or just one of us in shield and one in sword. For example, on the final dread in turn 4, a mob which I engage for a very short time, I consistently get to 80-90%% or more hate by the time it dies. Even if i was critting 100% of the time and he 0%, I should still not chase it down quite that quickly (as it has been alive for quite some time at this point, long enough to finish off dread 2, kill all associated adds in the final wave, and then for me to run acccross the room to get there). I really doubt it is just crit rates. Since we can both catch up to the other, I highly doubt it is owing to any skill difference between us (if one of us really was that much better at comboing, the other wouldn't be able to catch up). Similarly, this rules out any gear difference (again, we are quite close, within 1 piece of each other). Also, since our gear is so close, our crit rate is nearly the same as well, so even if it was crits, it shouldn't happen every time. We should, over time, balance out, yet I observe this every time. Be it me chasing him, or him chasing me.
CD's should not be a factor. Using a cd between combo steps does not delay your combo unless you are stacking several, but there are few/if any situations where this is needed and I already accounted for that in trials against Twin. More, in the phase of twin I am discussing, there is no call for the tank use cool downs anyway. Also, we should both be maintaining maximum uptime on FoF, so it should be the same bonus for both of us.