Whereas the Chemeketans, who were established
later, took advantage of the early 20th century advancements in
transcendental number theory to climb within the octodecimal
realm.
Despite Cliff's assertion above, my guess is that
Thielsen and/or Broken Top were both originally viewed as
relatively minor mountains or summits especially in terms of
"mass" or elegance, especially when it's the volcanoes that were
the big draws in terms of PNW climbing and having cultural or
geological significance. On the other hand, looking at some of
their various club badges and awards, it may just be that
there's a lot of fetishistic behavior in play. Auntie Ell may
have more pertinent (or accurate!) info as a long-time member of
both clubs.
On a side note, shortly after St Helens blew in
1980 and climbing was effectively terminated for quite some
time, there were a few efforts to remove St Helens from any such
climb/summit lists.