I observed that the list of contacts is sorted alphabetically, but by first name for some people and by family name for others, e.g.:
It would make more sense to sort them all by either first name or by family name:
What is the cause of this strange behaviour and how can I solve it?
It seems that most contacts are sorted by their family name by default. If a contact is sorted by its first name, it is always present as contact person in Microsoft Outlook.
I compared two people that were present in Outlook, of which (in Skype) one was sorted by its first name and one by its family name. I checked whether there were mistakes in the name fields in Outlook, but this seemed not to be the case.