I got a theory but it is only a theory and it goes like this:
chances of hitting a slay (making the numbers up now) is achieved if you pass a roll of a random e.g anything over 95 means the 0-100 random result only provides a slay if the result of the roll was 95+ however as each tick passes the 95 is reduced or the base roll is increased e.g the pass for a slay is now 75 so random 0-100 will provide a slay if the result of the roll is 75+ another tick goes by and the 75 is now reduced to 55 so random 0-100 will provide a slay if the result of the roll is 55+.
Now if the slay occurs the "pass number" is reset back to (in the above example) to 95.
The other part of my theory is to do with the slay undead and vanquish undead AAs - what I think happens (in my theory) is that the same random roll takes place 1 - 100 however with each AA the starting point of the number is increased e.g slay undead 3 AA increases the 1 100 roll to a 5 -100 roll vanquish undead AA increases the roll from 5-100 to 10-100 and anihilate undead from 10-100 to 15-100.
In my theory due to the pass number being reset to (in this case) 95 the chances of getting double treble quadruple slays e.g. one slay after another is highly unlikely.
What holyforge does is probably but I dont know is either increase the base again by say 10 but might instead reduce the pass number from 95 to 85 - I think it's the latter - so during the 5 minutes holyforge is up you will see more slays but no consistency in the number of slays.
With regards to fero and cleave etc... I have no idea how they work but I suspect when you count in all the AAs fero and cleave they all stack but the base random has a cap e.g. you cant when rolling a 1-100 increase the base number above (say) 30.
That's my theory and I think this mechanism is true of other things e.g. spell crits mellee crits weapon procs etc...
I came to the above theory to the many times where I have had attack on but been out of range and when the mob finally comes in range I get to see a slay in the first few hits consistently.
Now I am not one for proving my theory and how you would go about trying to prove it - I haven't the faintest perhaps root undead stay out of mellee range have attack on and parse the results compared to just not rooting the undead.
By increasing your haste overhaste etc... means that you have more rolls at getting a slay in a minute but as soon as you get a slay the pass number (95 in my example) is reset so you will over the longer run get more slays but it would be very difficult to work out how many more you would get.
Of course the random 1 - 100 maybe a lot more complex than what I have stated here and atk rating etc... might come into it too.
Well that's my theory of how slay undead works - someone can now come in and debunk it completely