I've been thinking about this and I don't think it ever comes up aside from a brief conversation between Shantay and Myfanwy, but Rook Thomas/Myfanwy and Bronwyn both pronounce her name differently.
It could be that their parents used both pronunciations, or that the 'rhymes with Tiffany' pronunciation was some kind of nickname, but I think there are two more likely possibilities.
The teachers/other children at the estate couldn't/didn't pronounce it properly and young Myfanwy couldn't or didn't correct them.
Or Myfanwy's family used the 'rhymes with Tiffany' pronunciation and after Myfanwy left home her family didn't talk about her to the extent that Bronwyn, who obviously adored her and desperately wanted to know where she was, who was one of the people Rook Thomas said she loved the most, didn't know how to pronounce her name.
Both of those options are just so interesting and sad! On the one hand young Rook Thomas, taken away from her family in a way that clearly hurt her deeply and losing the last shreds of her old life as even her name is altered, and on the other, young Rook Thomas, suffering and alone, while the people who loved her and were fighting for her were separated from her to the point where they couldn't remember how to say her name.
