It is most likely due to fact that Ryumma turned a regular sword into Black one, while the new ones are of better quality and has special abilities like Enma to suck in more Haki or stronger curses. So when they turn Black, overall they become better swords.
Afterall its Oda ranking em and he knows everything.
Yes, Oda is providing us that information and that's why it's important to look at all of the information that has been provided. IE: who was the one in the manga to rank shusui and did they know anything about a black blade and how it's rank. If I'm not mistaken, Tashigi told us the rank of shusui based on her sword book. At this time, how a black blade is ranked is coming from a swordsmith, Hitetsu. If nobody else in Wano knows how to rank a black blade than that explains a lot.
There could be more to Shusui then what we realize. When Shusui was introduced Zoro stated it had a very strong blade, it wouldn't bend even if stepped on by a dinosaur, and it was a heavy sword. Now the question is, is shusui that way only because of blacken haki or was shusui a naturally strong and heavy blade and haki just made it even stronger. We know swords can be different weight, Zoro said Yubashiri was a light sword and easy to use. Zoro has never stated blacken haki increases the weight of his swords. Yes, I know permanently turning a sword black could cause it to weigh more too. The point I'm make is this chapter creates even more questions that needs to be answered before we can say if Enma is naturally a better quality sword than Shusui.
This chapter ask the age old question, whats better to have, a stronger offense or stronger defense. I believe Enma is known for it's offensive power while Shusui is known for it's defensive power.