According to this example taken from CanJS Documentation (Bottom of the page):

• add and remove events are not being fired, only the length one

I believe we should have this output:

// remove ["eve"] 2
// length 2 3
// add ["Xerxes"] 1
// length 3 2

And not only:

// length 2 3
// length 3 2

FYI: This was already raised and discussed in Slack 😉