It's worth noting that Futari BL Original is massively harder than 1.5 to play for a score run as opposed to a survival 1CC due to having to learn vastly more aggressive pointblanking techniques, whereas it's quite easy to keep your distance, bomb frequently and keep the rank from spiking, so Futari BL Original is a vastly different beast of a game depending on if you play for a 1CC or play for a serious score. It's the same as with Batsugun Special - I assume the ranking here is just for clearing the first loop, as opposed to doing all the loops in Batsugun Special (which is harder IMO than clearing the original Batsugun which doesn't loop).
As someone who has
1CC'd Giga Wing without reflecting, I can tell you its difficulty ranking here is massively overstated. It's rated at a 9, just below a DOJ Black Label 1-All at 10, Whereas Futari 1.5 Original is at an 8 and Futari Black Label Original at a 5. I'd comfortably put Giga Wing at a 6 or 7 in terms of relative difficulty - it was my first arcade 1CC and the game only
really gets nasty at stage 4. You don't get any 1UPs (the only way to get one is to be doing really badly and have died twice by the time you get to stage 4), but it's balanced by having a large number of bombs available on top of reflecting as a survival mechanic. A lot of players don't realize that Sinnosuke is actually the worst shot type (weak spread and slowest movement); if you go for a survival 1CC with Isha or better yet Stuck (who has a strong piercing wideshot with second-highest movement speed), the game is much easier. Reserve your Reflect Barrier for survival purposes and during the recharge aggressively bomb if you have to (which also gives a guaranteed Reflect Barrier recover).
The first four stages tend to have relatively slow bullets and nothing really insanely difficult; the hardest part of the game is stage 5 which requires some very aggressive routing and memorization of enemy appearance so you save resources. The bosses after can be difficult, but not unfairly so and with a bit of practice and learning where the best bits are to reflect for safety, it's not unreasonably difficult to get the clear. Rank also isn't a factor (if there is rank in the game, it appears to be fairly minimal). You have fewer lives available in Giga Wing compared to DOJ, but you also are generally much less likely to lose a life if you're good at reflecting, whereas DOJ has some downright nasty stuff at stage 4 and beyond, and generally faster bullets throughout the game.
I admit I could just be biased for Giga Wing being easy due to having played it a bunch. I think it's a great beginner shmup though as the last few bosses do not have bullet speeds anywhere near DOJ, so I'm pretty sure my assessment of Giga Wing's relative difficulty is a fair one.
Eschatos at a 9 difficulty for Original Normal also seems out-of-place. Its Hardest difficulty would be well above this list's equivalent of a 10, no questions asked, but I'd peg Normal at about a 5 for a 1CC, Hard at a 7 to possibly high as 9 for a 1CC.