Disclaimer: this is just my understanding, maybe I'm 100% wrong. (Prove it!

)
So the first thing to understand is that every proc-gen name is resolvable to a specific set of coordinates. Taking PRAI HYPOO AA-A H60 as the example, the galaxy consists of 128x64x128 sectors and "PRAI HYPOO" will resolve to a specific x/y/z in the galaxy. "AA-A h" tells us the masscode and the location of the boxel within the sector (for mass code h, there is exactly 1, and it is the size of the sector). "60" tells us the number of the system, and that part can tell us the very exact location within the boxel that the system sits. Each LY in the galaxy is split into 32 - meaning a 1280x1280x1280 actually has 40960 x 40960 x 40960 possible points. Even an a-mass system can exist in a 320x320x320 set of points.
Others have worked out the first 2 parts - the system name and boxel ref decoding, but the exact location within the boxel still remains a mystery. That said, the coordinates are calculable by the client - it doesn't "ask the server" where a system is. I believe
this video explains the mechanism at work. Ultimately, it comes down to "simple" algebra - if you have a formula, say, y=x+1 - you can always solve y given x - and x would be the "60" part.
With me so far? OK. h-mass systems are the only mass code I have witnessed to start at non-0 system numbers. It is my belief that it's actually an artificial dampening mechanism - otherwise stellar forge would be generating h-mass stars in places far away from the core. We see other hand-crafted damping mehcanisms, so this doesn't seem like a stretch. Regardless of the reason
why, it is still a fact that some h-mass systems start above system number 0. In Horizons, you couldn't search to these systems, it is aware that they aren't there and won't take you. I believe it also won't take you to those proc-gen nebula with h-mass names, but that don't have a specific origin system. But it's an artificial limitation - the actual coordinates are knowable, and Odyssey will resolve both proc-gen nebula, and non-existing h-mass systems. Both HZN and Ody will not let you search past the end of a boxel - so you can't go to PRAI HYPOO AA-A H100000, but coordinates for that system
are calculable, and if the interface didn't artificaially stop you, it'd take you to some seemingly random (actually not random, see above) spot.
But there are other systems that take you to phantom spots! These are
all systems that have been tampered with by FDev (or "hand authored") if you prefer. They're usually (all?) catalogue systems that have ended up as secondary stars in a different system (but usually very close by). Both Ody and HZN will take you to these phantom systems - because the search is unaware that the specific systems doesn't exist - the system number is still within expected bounds, they are unaware of the concept of holes in the sequence.
IC 1805 Sector was also mentioned as having holes. I'm willing to bet that searching for whatever one of these "missing" systems is takes you specifically to proc gen system. The name IC 1805 Sector is
not a proc gen name, it is an override name. In fact, anything with "Sector" in the name is an override - though slightly different to other hand-authored systems (like the HIP catalogue). These "Sectors" are spheres centered on a specific point, with a specific radius. Anything within those spheres gets renamed. Being spherical, they cut through boxels at their edges - so you might have "procgen XY-Z d0", "procgen XY-Z d1" outside of the sphere, but "procgen XY-Z d2" ends up within the sphere, and becomes "name override sector AB-C d2". The boxel code in these instances changes - but the mass and system number do not. Searching "procgen XY-Z d2" will still take you to name "override sector AB-C d2" - because it's a procgen name and it's cordinates are calculable. I actually don't know what searching for "override sector AB-C d1" - which would be outside the sphere for that override sector would do - but it can only be 1 of 2 things: a) it does nothing because it realises the coordinates for that system would be outside the sphere of "override sector", or, b) it takes you to "procgen XY-Z d1". My guess is the latter.
TLDR: Odyssey pretending certain h-mass systems are there when they aren't is a UI bug.