Moin! Thank you very much for your message. Since the way you write the Low Saxon language suggests you studied it in Germany, I'll answer in English, although you'd probably also understand my native Dutch.
Before answering your question, I'd like to say that I'm thrilled to read that Low Saxon has second-language speakers!
As for your question, what you often see in historical linguistics is an abstraction of the situation. We often speak about Proto-North Sea Germanic as if it were a monolithic language, separated from the other varieties of Proto-West Germanic, and gave rise to three languages: Old English, Old Frisian and Old Saxon.
You could see the varieties of Proto-West Germanic as one big dialect continuum: a 'rainbow' spanning from modern-day Denmark to Switzerland. As in a rainbow, it's impossible to point out where red starts and stops, it's very hard to clearly demarcate the West Germanic dialects.
Instead, let's view Proto-North Sea Germanic as an area roughly covering the shades of red, yellow and orange. This conveys that there was variation within, while these varieties shared certain aspects of 'redness'. It also shows us that in the south, this reddish-yellowish area gradually transitioned into the greenish and bluish dialects that would become Old Dutch and Old High German.
As Old Dutch, Old High German and their descendants demonstrate, there are these transitional areas in the continuum - which still exists - where the regional languages have characteristics of both Saxon and Dutch or any other combination.
The ancestor of Old English used to be part of this continuum too, but as it was transported to the British Isles in the 5th century, it eventually really became its own branch, separated from Continental Germanic.
I hope I sufficiently answered your question. If not, please do tell me!
