It purely depends on your social circle. You get small communities where there are large numbers together. If you live near one you are much more likely to know lots. If you know one, you are likely to know a cluster by extension.
They aren't evenly distributed throughout the national population.
This also has the effect of narrowing echo chambers and limiting their own social communities.
Even when my brother announced he was trans in 2006, he was saying he was told that at the time there was a 1 in 10,000 representation. But that it was known to cluster. Another boy in his school class (his older sister was in the same class as me - I knew the other family really well) had come out as trans a few years earlier. Both boys new another who was in the year below them. In 2006 NO ONE was coming out, yet I knew three in my small community. And one of them was probably one of the most closely linked families to mine. It made no sense at the time.
For a long time this cluster stuff made me really curious in various ways. If being trans is just something that 'just occurs', then you wouldn't get a cluster effect in the same way. It would be much more evenly distributed. So why was it happening, even back then?
Had something happened to them? (there was issues with another kid in that class which I won't go into here - he later killed himself), was it genetic in someway (both boys were bright with what id probably call now in hindsight autistic traits), was there environmental? (chemicals in the water was a theory for a long time), was it social? (younger brothers who had higher achieving sisters and were not high in the social hierarchy) were they coming out because it was more socially acceptable in our social circle?(which suggests lower expectations of transphobia in the area), did they have similar social circles? (did certain groups move to a similar area), did they have similar interests?(the internet was definitely a huge factor with my brother, I don't know about the other kid but I know his family got the internet really early compared to everyone else I knew growing up), was there something else going on? (the other kids family was VERY homophobic and my parents were very rigid on gender stereotypes).
This was very anecdotal though.
As the years have gone on, these cluster driver have become a lot more apparent though - the observations by the Cass Review are interesting as is the very high numbers working in IT, having an interest in anime and gaming and the levels of autism. It's becoming interesting hard to argue there's not an element of social contagion /influencing going on. In which case is it innate or a social construction which reflects our time in history?
Yeah so clusters. Make of them what you will.