Ce sont les siennes
I would have expected it to be: Ces sont les siennes. Could somebody explain why the singular is used?
Certainly. This can be a little confusing since there are two parts of grammar involved here.
C'/Ce + the verb être are demonstrative pronouns. C'est = it's / Ce sont = they are.
Ce, cet, cette, and ces + a noun are demonstrative adjectives. Ce garçon = this or that boy / ces filles = these or those girls / cet appartement = this or that apartment.
Now if you change the sentence a bit and put a plural noun after "ce" you can have: Ces robes sont les siennes.
It's not only that "These are his (or hers)" but more specifically "These (grammatically feminine objects) are his (or hers)." It's « les siennes » because the objects they refer to are plural and feminine. If it were one (grammatically feminine) object, it would be « C'est la sienne » (« C'est » is a contraction of « Ce est »). If it were multiple (grammatically masculine or mixed) objects, it would be « Ce sont les siens ».
In English, possessive pronouns do not agree with number (or, obviously, gender) of what they replace, but in French, they do. The Possessifs-2 skill's tips has the rundown.
Now, why must you use « C'est » and « Ce sont » and not « Il est, elle est » or « Ils sont, elles sont » with these possessive pronouns? Just because that's how French is.