The Wikipedia article you refer to gives a reference which says that this change
"increased speed and stability". Which can be explained, of course. Suppose you
have two gauges, 1524 and 1520 with the same tolerance, say 4. This (very roughly) means that the first one will really have width from 1520 to 1528, and the second
from 1516 to 1524. ("Roughly speaking" because really there will be some probability distribution of the width, but we can consider a simplified model.
In any case, 1524 is only the AVERAGE distance. The true distance varies. Hopefully within the tolerance limit.).
Then probably 1528 is too wide for "speed and stability" of the existing rolling stock, and again it is easy to explain why. The wheel has inner rim which is on the inside side of the rail. This inner rim is needed for stability.
If the distance between these inner rims is much smaller than the gauge,
there will be an instability.
Shortly speaking the change can be justified exactly in the case when it is
difficult to tighten the tolerances.
On my opinion, this is a purely engineering question, and I doubt that this was
"Brezhnev's decision". Though of course he could stamp this decision, as a very important one for performance of the railroad system.
And the question is purely technical, hardly belongs to "History".