Is this just to maintain a safety buffer or is there some other knowledge to be had diving between 100' and 130' or is this just another way for the agencies to sell another specialty course?
Apologies if this has been covered; I've been skimming the topic the past few days. Others have pointed out you run a substantially escalating risk of performance impairing nitrogen narcosis going deeper. This is a serious enough issue to be worth expanding on.
1.) You may not realize you're impaired. Occasionally someone posts a 'funny' story where a dive buddy does something odd and doesn't recall it after the (deep) dive. Or someone seems to be wondering around apathetic and progressively low on gas. Being seized with an anxiety attack that resolves with maybe a 15 foot ascent, perhaps.
2.) The nature of narcosis effects varies. Whether you're just slow, functionally 'dumb' or dangerously indifferent or unperceptive, have narrowed awareness or an anxiety attack...if you don't realize it (or how bad it is) and you're rapidly running out of gas and NDL time (and likely haven't been trained to deal with deco obligations, even if you have the gas left to meet them)...well, things
could 'go south.'
3.) Narcosis effects aren't consistent person-to-person. Some people are unusually susceptible, some more resistant. Just because most people are fine with a dive is no guarantee you will be.
4.) Narcosis effects aren't consistent over time in the same person. I remember one of our seasoned members,
@drbill describing different experiences when he'd been diving regularly vs. when it'd been awhile. As a result, I prefer to avoid diving very deep my first day or two on a trip (especially if diving solo), and I don't go over 100-feet solo without a pretty good motivation.
5.) Depth and duration aren't the only factors in play. Others already discussed that.
I also got dark narked bad on that wreck. I did it on a HP80 slinging an AL30 with a buddy who wandered a bit. I got down to 117ft when we dropped off the stern to see the huge twin props. It was dark and bad viz.