Why "byse" and "vyse" and not "bies" and "vies"?

[squee100]

https://books.google.com/books?id=KEYEAAAAQAAJ&pg=PA17&dq=hebrew+vowel+shvo&hl=en&sa=X&ved=0ahUKEwiCpubS7YLhAhXtQhUIHcCCCAcQuwUIMDAB#v=onepage&q&f=false

Elsewhere in this book, the author consistently transliterates Ashkenazi צירי as "ie." Why, when listing the letter names, does he use "byse" and "vyse" instead of "bies" and "vies"?

If you'd use IPA, we might have a clue what you mean. I don't know what "byse" or "bies" sounds like.

The author says the sound is "ie" as in "die," so [bajs], [vajs].