Visit Standard Length and Average Visit Duration

On my site, we’ve had ‘visit_standard_length = 1800’ set as normal and see around a 4 minute average visit duration.

After setting it to 0: ‘visit_standard_length = 0’ the average visit duration jumped down to below a minute.

After setting it to 18000 (300 minutes): ‘visit_standard_length = 18000’ the average visit duration jumped up to around 13 minutes.

My question is why does changing that parameter alter the average visit duration, or is it even related at all?
This was the only change made and our average visit duration has been consistent for a while. Making the changes caused a noticeable change within hours so it seems like they’re related.