farms-production time unusual

[Asutsa]

why is the distance for the middle farm to the field 8 and not 4 like the other 2?

[Landarean]

Because the farmers work a different field as soon as it's exhausted.

Meaning it's very well possible the farmer in the middle isn't working the field in front of it, but can be on the outer one.
So instead of walking one field, he's now walking 2 fields to get to it. Hence the double time.

All harvesters are on different delivery times every time due to the distance to various deposits.

Hope this helped.