I live in the UK and my first moulds came through our NHS along with my hearing aids. I then had a set of moulds made privately and when I got them I found that the new privately made moulds fit further down my canals and the moulds are generally a tighter fit in my ears than my NHS made ones.
You are right though the sound quality is a lot better with moulds that go deeper into the ear canal.
The one on the right looks like it belongs to Prince. Or maybe it looks like a slug that just came back from a hike in Chernobyl. J/K; actually I like the look of dark inside the ear as opposed to plastically looking fake skin tone things
I’m originally from Surrey England and had always used the NHS Audi’s. I completely understand what you mean by the fit, I didn’t know any better till I moved abroad and purchased my first set of private molds. I was blown away the fit it was perfect no soreness it was almost like NHS molds were made deliberately far bigger to force the mold to stay in, or a mold of the mold was made so the details were lost. I’ve never had long canal parts mine have always been sort of short I just assumed It was because I have small ears ( my molds have always been small)
Have to say I LOVE the glitter look of them I’ve never been brave enough to go for bold colours always stuck with the boring flesh or soft pink. If I could choose or had the boldness of some users I’d go for glitter in teal or kingfisher green (my fav colour)
I expect one of the learned Audiogists to jump in here to provide the answer.
In the meantime, my experience suggests that the longer mould both reduces the cubic volume of the canal that needs to be filled with sound and also places the source of the sound closer to your eardrum (as seb suggested above).
Perhaps we can visualize the process by considering the size and power of a speaker to fill a closet against the size and power required to fill a theatre with sound.
I have large sized ear canals that need my amplification to be increased via the REM fitting method.
I also have found that the hard acrylic moulds to be far more comfortable to wear and less irritating to my canals than the soft type.
Mine are hard acrylic and are so comfortable despite having them double dipped. Having the longer ear canals has worked wonders with keeping the ear molds in my ears. Normally I’m having to keep pushing them back in every time I move my jaw (eat/talk). They never came out completly but came out enough, to get loss of sound which was always annoying.