I was checking a recently scanned Japanese book from the 19th century about the American War of Independence and it struck me that its style and especially its depiction of the human form is basically the same as say, a (NSFW) pornographic book, they both are from the same century, yet I often see this style associated with classic Japanese visual art in general, is there a reason this particular style was so predominant?

Why would someone drawing a historic account and someone trying to draw raunchy erotica go for the same artstyle and what are the cultural reasons for it to have become so predominant to the point it seems its the only artstyle Japanese artists seemed to use from a layman perspective like mine?

Followup if possible: the previous books represent people from vastly different ethnic backgrounds (Europeans and Japanese), yet they are both drawn in the same artstyle, is there a particular reason Japanese artists decided this was the way humans in general should be represented?