It is indeed cut the way you describe.
It looks like a little too simplistic heuristics here that cuts stuff prior to first "is" or "are".
I've seen this work well with link-only-answers tag at Meta Stack Overflow where it cuts off "answers that are" and displays excerpt as "barely more than a link to some URL".
Workaround is to reword tag wiki excerpt to something like:
salary is how much someone is paid
Above should cut excerpt at first is but the remainder will look reasonable: "how much someone is paid".
I am calling this "workaround" though maybe it's the way how things are intended to work - searching through tag-wiki-excerpt questions at MSO did not bring me any information on that.
salary wiki excerpt has been changed as proposed above, so that it is now rendered better: