Why "last" and "new" are stopwords while "past" is not?

I ran the following texts through Rake(default settings) and their output are included. "new" and "last" are not included but "last" is included. Just wonder how Rake makes decision on this.

Text: "new accounts opened in last 6 months" --> Rake output: "accounts opened","6 months"
Text: "number of contacts in past 12 months" --> Rake output: "contacts","past 12 months","number"