Is it? Are you familiar with DC-9 and how many seats were in each variant and who operated them? The numbers aren't that dissimilar from RJ's of today. Along the same lines, ERJ-170/ERJ-190 type rating covers the 170 through 195. Up to the company which variant/seat configuration/metal they want on certain markets... Kinda like how we do it here with -700's now with 137 seats vs. MAX 8 or -800 with 175 seats for the same rate. On a side note, we also used to fly the -500 here (my favorite Classic by far) and it only had 122 seats, and it paid the same as the -800.

Lastly, I'd pay attention to what guys like Zap have to say... Eeyore (Zap) has been around the block and probably has more airline uniforms than most normal people have underwear.