My error. The Soviets have to pass admin checks to get resupplied. Must have been thinking of a different game. Here's the rule. In 41, they have to pass two admin checks; in 42, on admin check.
The point is still the same. A truck shortage lowers the amount of supplies units get. Due to the admin checks, the shortage gets higher or happens more often. This results in more supply checks, and less supplies getting to the front. Which then requires more supplies the next turn. Which requires more trucks.
For example, if the admin rating is 7; in 41, that requires two checks so the probabity of getting supplies in 41 is .7*.7 = 49% chance of getting supplies in 41. So fewer supplies in the units which means more trucks carry supplies the next turn which means more damaged or lost. And so on.
220.127.116.11. SOVIET AMMO SHORTAGES
To simulate Soviet ammo shortages in the early part of the war, Soviet units in 1941 and 1942
must conduct a leader admin check when resupplying their units with ammo. In 1941 they
must take two successive checks, in 42 one check. As ammo is distributed, if the check fails,
then 50 percent of the ammo is lost during conversion from supplies. If both checks fail, 75
percent is lost.