Just a wild guess, but perhaps it's part of a vowel shift either in modern Greek, or modern English.
I know UK (bee-ta) and USA (bay-ta) pronounce Beta differently but am slightly ashamed to say I don't know how modern Greeks pronounce it. I understand it's derived from Hebrew's Beth, though?
"In Ancient Greek, beta represented the voiced bilabial plosive /b/. In Modern Greek, it represents the voiced labiodental fricative /v/."
For centuries, and probably still today, the language taught at school was Ancient Greek -- that's where I learnt the pronunciations. I don't know if pi had a similar change in Greek pronunciation.
I know UK (bee-ta) and USA (bay-ta) pronounce Beta differently but am slightly ashamed to say I don't know how modern Greeks pronounce it. I understand it's derived from Hebrew's Beth, though?
Language is a constant curiosity.