The explanation seems simple. The national championship was held only every two years.
Excerpt from "Asia-Oceania Soccer Yearbook 1986-87" (Paul Moon and Peter Burns):
"The biennial National Championship again proved the superiority of military sides with the capital city's police representative Dynamo Ulan Bator (or "Khuch" to give them their local name) always looking head and shoulders above the other competing teams in ability. Friendship Darkhan (representing a Soviet military base) and Labour Ulan Bator finished as runners up and third place getters respectively.
In the Ulan Bator City Championship, second only to the National Championship in importance, the Suhbaatar district selection were victorious.
and from "The Asia-Oceania Soccer Handbook" (1985)
"Until recently, Soviet troops were the sole participants in Mongolian domestic soccer, therefore little information on activity leaked out. Today native Mongolians are involved, but still few details escape the country.
It is known though that 72 teams contest the National Championship. Initially they are divided into 6 regional leagues, each containing 12 teams. The half-dozen league toppers then contest a two round playoff to determine the overall champions.
Mongolia's best performed clubs are Adlar, Khudulmur, Khorshoolol, Khuch, Kerulen, Orhon, Bugun, Hentiyn, and Haraa."