You're being pedantic in muddying things with extra variables, namely the adjustment due to full justification. Let's simplify things and just consider ragged

You're being pedantic in muddying things with extra variables, namely the adjustment due to full justification. Let's simplify things and just consider ragged right. In this context, TeX is indeed approximately 1.5 spaces after a period, except for \frenchspacing, which uses 1 space. AFAIK, the reason TeX uses this default spacing is because Don thought it looked good.