As I battle my way through DLC East 1942 I noticed the amount of heroes awarded were low, as compared with 1941... But as I started the first scenario in 1943, I noticed that 90% of my units taking a shot, be it infantry, tanks, etc. suddenly are rewarded with a hero. With whatever result of 0, +1, +2, etc of the predicted outcome.
I saved each outcome and then played the same turn about 10 times to check - with each time the same result, though with differing heroes awarded. I ended up with 14 heroes rewared in one (the first) turn  
 
Obviously having to do with differing settings, but which ones?