A bit of both probably - before records became more widespread, speakers of the oral European languages had a tendency to shorten words - when languages were written down the existing pronounciation was interpreted by the scribes from the closest sound they knew. Some words continued to be shortened and we now have the situation in many local dialects throughout the British Isles of fairly long and complicated words being pronounced as short words, largely unrecognisable from the written form - e.g. in Cheshire there is a village written Cholmondley but pronounced Chumley.

With the Gaelic languages, before surnames were needed by tax-collectors, lawyers and record keepers individuals were identified by mac (son of) (or ine for daughter of). In the Isle of Man this identifier was transformed into surnames when surnames were demanded by the Island's rulers in 15th century. Over the next decade the natural tendency to reduce words into the shortest unique form produced the C,K and Q surnames which were then written down in taxation records and parish records - once surnames were written down they largely remained unchanged, although there are some exceptions.
... so, linguistic laziness - probably compounded by foreign scribes unaware of local culture. I was hoping to find a more interesting process for the name formation - and still am, but this is probably just wishful thinking.