Explanation on ADD and REPLACE

Can I have more information about the chance calculations for the ADD spawn function compared to the REPLACE spawn function? It seems with ADD that either the mobs are spawning WAYY too much like within seconds I am surrounded by the custom mob, but if i switch it to replace, the mob is nowhere to be seen?

I understand, from the documentation, that the REPLACE function takes the mob type and has a chance to replace it with the mob specified. So if i make a custom cow called toughcow and...

Explanation on ADD and REPLACE