Hello!The reason why it’s always transcribed /ˈmætərz/ is because of the special rule associated with /z/ and /s/. They have unique rules when a word ends in “-s”. In the word [matters], the grapheme following the [s] is voiced, so we would use /z/ because it is also voiced. Voiced/Voiceless graphemes will be followed by the same type.For an opposing example, the word [cats] ends with a [t], which is voiceless, so it would be transcribed with an /s/ at the end instead of a /z/.

I would recommend saying [matters] with the sound the /s/ makes in [cats], then say it again with the /z/ sound instead, and hopefully that will help it make more sense!