A run doesn’t have a set value. Instead, its value changes from one match to the next and arguably even fluctuates over the course of a single match. This is why it’s important not to pay too much attention to averages.
In Tests, the value of a run is defined by how many of them can be scored for the loss of 20 wickets. If a team can score 800 for the loss of 20 wickets, runs are plentiful and less valuable. If a team can only score 300 with their 20 wickets then a single run contributes a great deal more towards a team’s cause.
Similarly, in the shorter formats some matches are higher scoring than others. A run in a very low scoring Twenty20 match can be a very major stride towards a winning total, while in others it can basically amount to treading water as the remaining number of deliveries diminishes.