Wikipedia has some facts about Plutonium:
each gram of 238Pu spontaneously generates 0.568 W of heat.
There is 3,478 g of 238Pu at the start of the mission. The product of both numbers is 1975.5 W. That fits well to the 1,975 W in the document.
Some sources you found simply rounded 1975.5 W to either 1950 or 2000 W.
There was an early design goal for the MMRTG of 125 W electrical power. For the rovers Curiosity and Perseverance both :
approximately 110 watts at launch with little decrease over the
The electric power of the thermocouples depends on cooling influenced by the environment. On a cold day on Mars it may be 120 W, on a warm day only 110 W. (At the beginning of mission).
The Perseverance rover will need about half a year to Mars. The half-life of 238Pu is 87.75 years. After 0.5 years 99.606 % of 238Pu will remain, the heat power will drop from 1975 to 1967 W. 13.7 g of the 238Pu will be decayed during the flight.
But the decay of 238Pu does not start at launch, 238Pu starts to decay when it is synthesized. If it waits 5 years for launch, 3.87 % of it is decayed. Thermal power will be reduced from 1,975 W to 1,898 W during this time.