It looks like the 48 liter size describes the maximum volume of the pack, while the 35 liter name comes from what REI considers the actual useable volume. When pushed to their limit, top-loading packs have the ability to be quite overloaded, which is the main factor in this max vs useable volume difference.
The volume in a pack's name is also derived from its most common size (medium for men's packs and small for women's), though capacity does slightly change based on which size you have. REI's measurement for max volume on the men's large is 46.5 liters, which isn't too far off from Outdoor Gear Lab's measurement.
Thanks again for bringing this question to us! Was a fun one to get to the bottom of!