He was called "Leonardo" at the time, and if anyone asked "Which Leonardo?" then the reply could be "Leonardo from Vinci" (there are other combinations to identify someone, re sister comment).
Think of it as "Leonardo from Chicago". The title wouldn't sound right with "by from Chicago".
Icelandic names today are almost the same - the "daughter/son" part is more like a "from" identifier than an actual name. Thus you address an Icelander by the "first" name. Or you'll end up doing the equivalent of calling someone "from Chicago".