The relatively poor print sharpness of 20-25 line pairs/mm is caused by “sideways image spread” during diffusion (a thin layer of developer separates the negative and print, which are not in perfect contact). The spectral sensitivity may reflect the fact that Polaroid Type 55 was introduced in 1961 and has probably remained exactly the same ever since. Polaroid’s first panchromatic film came out in 1955, so their knowledge of dye sensitization in 1961 may not have been cutting edge. The relatively low negative Dmax may reflect the migration of silver to the print.
IMO, Type 55 is a case where the observed results and benefits of its processing depart from seemingly outdated technical specifications. I've taken some of my best photos with Type 55, including 1:1 macro still lifes with available window light to broad landscapes. For a film that really has no variation in processing, it always works, and in many cases, works better than more "modern" emulsions and processing. I cannot describe in technical terms why that is so. All I know is that it worked extremely well under any conditions I could come up with. Sorry for the rant.