Counting the number of repetitions of the word man bash?

Question

linux

I find top 3 words by number of invocations in man bash
Script:

#!/bin/bash
man bash | egrep -io '[a-z]{4,}' | tr '[A-Z]' '[a-z]' | sort | uniq -c | sort -nr | head -3

First conclusion:

570 command
565 shell
336 value

But after some time, if the same script runs, the values ​​may increase upwards:
The second output:

571 command
566 shell
336 value

What could be the problem?