It can sometimes just depend on how well you apply. The authority I work for has a very strict interview selection policy which depends on the applicant showing, on the form, how many 'essential' and 'desirable' criteria from the job/person spec. they can meet. The more the applicant can meet, the more likely they are to get an interview.
There is a tendency, especially if you are already familiar with the selection panel, to assume that they will know which criteria you meet and how - and they probably do. However, they can't be expected to know this for external applicants and so it wouldn't be fair on the latter. Therefore it's vital that you address every aspect of the job/person spec on your application form, and it could be that this is what you didn't do on this occasion, hence the reason you didn't get an interview.