So for sleep, the spell goes up by 8 hp for the upcast from level one to two (24 hp to 32 hp). I suspect rather than go with an average value of 5d8--(5+40)/2=22.5, round down to 22--for the spell, they just reduced the maximum by 16 and then increase the spell's affected hp by 8 for each level of spell slot above 1.
If they went with average, it would be 22 for the level one version, then 31 for the level 2 version, and so on, adding 9 (the average roll of 2d8) every time.
I don't understand exactly why they did it this way, but seeing as how level 4 Gale can knock out two goblins in one go with level 2 sleep, it may be for balance? If they actually did the die roll, he could knock out the entire goblin group at the front gate to their camp, or most of them at least.
Also where would they put the die roll? If they put it before you cast the spell, then you could just cancel the casting and reroll. If they put it after, you wouldn't have any clue how effective it would be (which is how this goes in 5e but usually you can modify your aiming after you roll the HP dice if you have a normal, decent DM).