It's really confusing with the HIT points: they are a variable value...
In short: how to understand this
Added after 14 minutes
For example, we have 100 imps. We look in the imp table, or the monster table with 4 HP. Then, row by row, we look for the most advantageous number of imps. We find the number 97 and go up one row to DEMON and find the number 11. That is, from 97 imps we get 11 demons.
Then we look at the lower table. To get 11 demons, we need 8 Pit Lords. That's all there is to it :-)