Is it there true that flights can be cancelled because of poor seat numbers like e.g. 110 seats only 7 people on it?
The only times I have seen cancellations such as this are when airlines are in a dodgy position financially or are about to close a route and are not trying to increase passenger loads on that particular route. Yes, it happens, and the reason given is invariably covered by another issue, "weather", "ATC delays", "mechanical" etc, etc. Airlines have to account for cancellations, and would get into a heap of trouble if they overtly cancelled flights solely because of poor sales.
Airlines don't normally cancel flights unless there is a significant effect to the operation. It can be due to mechanical issues / weather / crew issues, or any combination of these. Examples can include such issues as a crew member calling in sick, with no-one available to cover; or a delay causing the crew to run out of duty hours before they can complete their rotation. In this situation, if they cannot recrew the aircraft, it could result in a delay of the return flight until the following morning to give the crew the required legal rest. However, this could also have a big impact on the next day's operation with crew and aircraft out of position, resulting in a knock-on effect all day.
What they typically do in this situation is look at cancelling the flight with the least impact on the airline operations and the bottom line. So they would look at the best way to protect their overall operation - be that cancelling the round trip with the lowest loads and/or lowest overall yields, or the round trip that would have the worst effect on the overall schedule / crew routings and/or crew duty time. It is complicated, it is messy, it is very fluid and the airline is often dealing with a number of variables which can keep changing.
If they have a flight with a low passenger load, logically that would be the one cancelled. However, say the low load was all businessmen / frequent flyers, they may well operate that trip to protect the passengers who are likely repeat passengers - even though they would be cancelling a flight involving far more passengers.
The other part of this equation is what happens to the passengers affected by this cancellation? I know with Ryanair, etc, that a cancellation would result in an eventual refund, and likely to have to pay the going rate for a ticket home on another carrier, which could result in significant costs as you will be paying the current walk-up ticket costs, not what you paid two months ago. Or, if they can rebook you on another Ryanair flight, but it may be a couple of days from now (and who pays for your hotel in the meantime). I believe that Wideroe's are a full service carrier, so I presume they have an agreement with SAS or other carriers such as KLM, where the affected passengers would be rerouted somehow at no extra cost to themselves. Bergen - Copenhagen - Manchester, or Manchester - Amsterdam -Bergen for example.
Cancellations are an unfortunate fact of life, and can be a major headache, particularly with the low cost carriers; or they can be an inconvenience if handled well by the carrier involved. If handled well, you may well keep the goodwill of passengers and they will fly with you in the future; if not, then you may well have lost them to other carriers in the future. My carrier has all sorts of algorithms to help the operations planners make the best decision in these situations.
Sorry for rambling this past couple of paragraphs, but hopefully I have addressed some of the issues surrounding cancellations in general. I deal with them frequently from an operations point of view, and my better half deals with them at the gates and ticket counter. One of the reasons that I much prefer my job to her's because I don't have to deal with the unfortunate passengers who are thrown into this situation. I am sure that Garstonboy and I could spend a whole evening and several beers reliving some of the horror stories we have had to deal with over the years.