YANBU, it is confusing!
The reason why though is because the different info is coming from different sources.
RF to age 4 comes from safety experts / Sweden / longer term research and is based on eliminating instances of death or serious injury caused by internal decapitation/spinal cord stretching injury caused by the massive forces present on the neck in a forward facing harnessed seat. If you look at the data it is not exactly 4, it's 3-and-some-months but for clarity's sake, it's 4. But those FB groups where somebody has a child who is 3 years 11 months and they are advised that it would be horrifically unsafe to forward face them now... are taking it a bit too literally. Guidance has to be simple and 4 is simple.
RF to 15 months is slightly different, if you look at real world crash data (crashes where at least one person was injured) and look at child occupants and whether they sit RF or FF and whether they are killed or seriously injured in crashes, if you were to draw a line across where a change in legislation would save the most lives, it's 15 months. And that's why it's now the minimum for the newer regulation car seats.
Forward facing from 9 months goes way, way, way back. The slightly older car seat legislation ie regulation 44 was originally drawn up in the late 70s, early 80s. Car seats existed but weren't especially easy to understand, they wanted an easy to understand system - your child moves from type 0 (infant carrier) to type 1 (toddler seat) to type 2 (high backed booster) to type 3 (backless booster) and they wanted these to be easy to understand and it to be easy to understand which stage of seat you're looking for. So they built in each group based on average age and abilities. For group 1 toddler seats it was assumed that if the child is able to sit up and support their own head this is safe, they don't need the reclined position for sleeping as in group 0 baby seats. Originally, Group 0 seats only went up to 10kg, 9kg is about the weight most babies learn to sit so there it is. The next transition point which is 15kg is based on the average weight of a 3yo as it was generally thought 3yos are mature enough not to fiddle with a seatbelt. Obviously there is some overlap between each group to allow for differently sized children, but originally it made sense.
At some point they extended Group 0 to allow Group 0+ which is 0-13kg. But Group 1 seats are still from 9kg because it didn't make sense to change that classification, the recommendation changed though from "At least 6 months and able to sit up" to "around 9 months" and more recently to "at least 15 months" following the newer regulation, where it's actually illegal to use the seat FF before 15 months.
Car seat manufacturers - they need to survive as a business, and parents overwhelmingly want forward facing seats. Most people simply don't like the idea of rear facing. It's perceived as complicated for the parent, unpleasant for the child, annoying to use (limited space, can't see child without a mirror). Put simply, purely RF seats do not sell, unless you're selling in an environment such as Sweden where RF for group 1 and sometimes 2 is the norm. So of course they produce and market what will sell. And just like baby food, if your product is usable "from 15 months" and your competitor's product is usable "from 9 months", which seat is the parent of a 12 month baby going to buy? Especially if the packaging and/or shop assistant assures them that forward facing is safe.
Recently, ERF has become more popular and so many companies have begun producing ERF seats even when they never did before. It's now easier and more affordable to RF than ever. But parents generally still want to forward face. If you try to sell them a RF only seat you can see they don't want it. Most people want at least the option to FF later. But I do see loads more people RF for longer and I think that's great :)