The routeings are not set in stone altough they are governed by the RAD (Route Availability Document) which tells you which routes are available dependent on area of origin and destination. It's not a user friendly document, you can have a look at it here
http://www.cfmu.eurocontrol.int/cfmu..._page/rad.html if you are bored!
Operators normally file the cheapest or fastest route depending on their priorities. If there are significant delays on those routes they will look for other routes with less delay.
Increasingly route charges are playing a role in route selection, for example Belgium is comparatively expensive to overfly at the moment so routes through Belgian airspace may take second place to longer routes through cheaper airspace.
There are others better qualified to give you more information about the many factors affecting route selection. Try the "Flight Ops, Crewing and Dispatch" forum or "Airlines, Airports and Routes"
