What you
believe is not the point
It's actually a fact: The Normans of the Normandy were the ancestors of Rollo's vikings, who were the last ones who invaded France at 911 AD - they got then the Normandy (first Rouen and suroundings, i believe; "Normandy" -> land of the Normans) as fief, in replacement to not make any anger anymore in west-frankish realms (but to defend it as vasalls of the french king). And they were "absorbed" quickly by the frankish culture, while still keeping the one or other viking traditions, but
not anymore the viking seafaring/raiding. The Siculo-Normans though kept and developed their seafaring abilities still more (as a remaining isle-conquering people, and with the growing connection to greek/italian/muslim naval knowledge). It is recorded somewhere, that the Siculo-Normans were the ones who showed the how-to, and thus managed, crossing the channel, of William's large invasion army with heavy war-equipment and the many horses, even in such dangerous waters like the channel. It was really an amazing undertaking, never before (and never again in history, besides the trial of the Spanish Armada), such a large heavy army landed on the british isles, including that many horses.
As a side-note: The Siculo-Norman house were a family branch of the Normans of Normandy themselves, thus i said "their cousins".