Excerpt not added if it's a substring of content

[scriptype]

I call stringify like this:

matter.stringify({
  data: { whatever: 'lorem' },
  content: 'this is a new page',
  excerpt: 'a new page'
})

and the result doesn't have an excerpt:

---
whatever: lorem
---
this is a new page

If I change the excerpt to a mew page (note the 'm'), then result has excerpt:

---
whatever: lorem
---
a mew page
---
this is a new page

So, why https://github.com/jonschlinkert/gray-matter/blob/master/lib/stringify.js#L46 exactly?