Pretty sure the fact they have less stored energy has a lot to do with it.
I noticed the same thing recently with a 68” Fox Triple Crown that I bought used. Initially, it was the loudest longbow I’ve ever shot. I don't know if that's why it ended up on the used market or not. But other than the noise, I liked the way it shot, so I experimented with different silencers and a set of limb savers, which didn't make much difference.
Then I let a friend of mine shoot it, who has an inch less draw than I do, and it was as quiet as a longbow should be! I tried drawing it an inch less, and it was that quiet for me too. So less stored energy = less noise. But a longbow should be just as quiet at 28” as it is at 27”, so I kept experimenting with it. Eventually, it occurred to me that the endless string that evidently came with the bow from Fox might be noisy, so I replaced it with a Flemish string that I made myself. This quieted it down to an acceptable level, but still not as quiet as it is when shot at a 27” draw.
I thought I might be losing some speed with the Flemish string, but it comps within 1-2 fps of the same speed as the endless string, and I have the same gap, as far as I can tell, at 50 yards. The endless string was twisted up quite a bit, which could have contributed to the noise. I don't have much experience with endless strings, but they say you should put as few twists in them as possible.
Anyway, it's quiet enough now that I’ve quit worrying about it, unless something else occurs to me to try.