As a generalised summary, I would offer the following ...
Scrambled weights from Murano are undoubtedly based on what has been made in large numbers in Murano since the 1830s/40s. And their roots go back (at least) to surface-decorated glass beads and other items which were a main produce of Italy since the mid-15th century [ref the book Paperweights by Sibylle Jargstorf, amongst others].
A major difference between 19th century paperweights from Italy (Murano) and those of other countries in the same period was that the Muranese items did not change very much in style until the early 20th century. And even when the chages did come about, the basic concept of the close-to-surface decorated weights remained a primary feature - and that is probably still true today.
What really changed in Murano in the late 19th and early 20th centuries was the use of canes and filigrana in brighter colours and with clearer definition. The addition of such as aventurine edging to the filigrana was also another step in the direction of brightening things up. However, those "new" concepts were used widely in Murano and it would hard to pin down any particular maker or designer as an originator of those changes. Perhaps Salviati & Borovier had some strong influence, although that does not mean that scrambled weights were "designed" by them specifically or by any other glass master in Murano.
Its simply comes down to a very old technique that has been modified over a long time.