The first extended rounds were done before anyone thought of what might be suitable or valid peaks for doing so.
Applying a set of rules "after the fact" might have meant that existing 50@50s be regarded as invalid. As a result some extra peaks might be thought of as more deserving than others.

It does mean that the standard 42 is defined and the 24hr record also has a set of defined tops but everything in between doesn't.

I'd imagine that Paul Murray and Dennis Lucas simply built on their 50@50 rounds (but have no evidence to support that). Incidentally Paul and Dennis are the only members of a very, very exclusive club to have done a standard round, a 50@50 and a 55@55.

I'm still waiting for Julie to get back to me with her list - she's had a computer malfunction - so it may just be coincidence that the five extra tops for the 55 are the same for the other three.