boy /bɔɪ/ 和 oyster /ɔɪstɚ/亲前1个词的y发/

boy /bɔɪ/ 和 oyster /ɔɪstɚ/亲前1个词的y发/ɪ/音时发/e/这个音?
但第2个单词的y是发的/ɪ/这个音求y的发音规则.,为什么发音不一样?

ai:rain laid afraid wait bait aid aim braid\r\n\r\nal:walk talk chalk call ball wall portal pedal mental continual fatal\r\n\r\nar:car far sarcastic bar sharp smart care bare square share dare altar star retard regard arm hare flare glare\r\n\r\nare:care bare square share dare hare flare glare\r\n\r\nair:chair fair airy aircraft hair pair repair seaair fair despair\r\n\r\nea:eat meat steal cheap lead flea clean tea release clear\r\n\r\nee:feed feel steel sheet seed feet cheese glee teeth flee tree bleed\r\n\r\near:dear hear near clear year ear tear fear bear \r\n\r\nir:bird flirt skirt shirt thirty first sir dirty stir\r\n\r\noo:too zoo goose loose shoot food door tooth bloom gloom\r\n\r\noa:boat coat coal goat goal load boa soar croak coat float\r\n\r\nou:out loud shout house mouse round count tour tournament pour soup bouy hour\r\n\r\now:know show row throw own yellow tomorrow cow how owl bow blow tow towl\r\n\r\noy:boy enjoy annoy coy boycott oyster boy toy soy destroy \r\n\r\nor:for nor worn torn corn horn editor vistor professor short doctor donor contributor sailor tutorcurb\r\n\r\nur:turn hurt urgent hamburger turn curtain murder Thursday fur blur turf surf \r\n\r\nck:clock block cock luck buck truck duck brick click hack crack track stack pack\r\n\r\nth:with farther these this weather bother though Thursday three thief thank growth thrive 还有像froth,fifth……\r\n\r\nsh:dish cash lash bush gosh shift shock shrink sharp smash crash shine shrine flash clash slash\r\n\r\nph:photo physical phone phrase physics philosophy triumph trophy phantom\r\n\r\nwh:whose wheat whole whether who what whisper whether whorl white\r\n\r\nch:teach cheap catch lunch charge check change choose cherish chalk clench much scheme choice \r\n有的可能不认识,挑着你会的写吧\r\n我可费了半天劲的喽!(^-^)