Each time a spawner block is opened to make changes to its data, the info it contains has changed. So, for example, If I set:

minDelay 0
maxDelay 20
spawnDelay 20

Each time I open the spawner for editing prior to scanning, spawn delay has changed from 20 to a random number. For example 6. If I close and open again, it changes again - for example to 12. Each time it is opened, the number changes. Is it supposed to do this?

Yes, spawnDelay is the delay for the next spawn attempt. That is basically the internal clock.
Along with the total number of spawns (which decrease if positive till reaching 0 at which point the spawner block will disappear)
Those are the only values that you should see change automatically.