Soil/waste pipe question

[stratplus]

Question I was asked today:
How come you can fit a kitchen sink waste straight from the trap to the main soil waste directly underneath but if the waste is in a different position like in the corner of the kitchen it needs a soil/vent/durgo?
Same situation with a toilet. Some, (bungalow for example) the toilet goes straight into the soil in the floor via a bent pan connector but when the soil had been brought into the corner of the bathroom it is done via a vented/durgoed stack?

 

[solutions]

I believe it is likely due to the height the waste will fall and how much air will be displaced ahead of it and sucked in behind it. Hydraulic jump and all that.