Triptote and diptote

[Tanweer]

I am refering to page 27 of the Handout of book 1.

Muslimatun (feminine singular) has dumma tanwiin at the end, and it is Triptote (tun, tan, tin).

Muslimaatun (sound feminine plural) also has dumma tanwiin at the end, but it is Diptote (aatun, aatin, aatin).

I had an impression that words ending with dumma tanwiin are always Triptote. I must be wrong, but can someone explain this to me ?

Note: I have only learned upto Lesson 17 of Book 1. I don't know if this issue is explained in a later Chapter.