Airlines tend to work like this: at the start of the "season" analyst define the margin for each seatbof each departure. Let's say that for Sunday flight from Venice to Hamburg, Economy, your target is 112€. (For simplicity I will discuss only a direct flight, but the same idea applies to any leg of any Itinerary).

Yield manager for that area/period has now the task to make sure he gets 112 or more on each ticket. And take in account that an unsold seat gets 0, so lowers the averge margin (which is what the yield is calculated upon).

This will soon make you realize that any chance to sell again a newly vacated seat is a boon.

So in the case of the OP the company can either assume that it was a honest mistake and he will somehow miracously get there in time to get on the second flight (3% chance?) or assume that he decided he does not care anymore, he had a serious accident, got fired, won the lottery, whatever (97%) and promptly put the seat back on sale.

The problem with a-b-c costing less than a-b is less obvious, maybe, but it has similar causes: for the airline it is more efficient to sell you the itinerary with a stopover so their pricing reflects that.

There have even been attempts to take passengers to court for getting off at the intermediate stop (they were dismissed) so it's definitely not just because the airlines are throwing a fit if you decide to change your plans.

Probably the a-b leg and the b-c legs sold alone are not very popular so they want more money to maximize the yield, while everyone wants to go a-c and return.