A question about boy and buoy's pronunciation

the phonetic symbol of them are both “bɔɪ”, but their pronunciation are different, why?

Thank you!

The pronunciation is identical.

please visit

their pronunciation are different.

Please visit:
oxfordadvancedlearnersdictio … ionary/boy
oxfordadvancedlearnersdictio … onary/buoy
where the way the icon has been set next to the phonetic pronunciation may make things a little clearer.

Then look again at

You will see there are two loudspeaker icons there, along with two sets of phonetic symbols.
The second icon matches the second phonetic symbol, and this is the phonetic version have indicated in your original post. It is identical to the pronunciation of ‘boy’

It seems that Merriam-Webster, being an American dictionary, put the American alternative pronunciation first.
Being British, I’ve never heard anything like that pronunciation in my life. It made me laugh!

In summary,
There are two ways to pronounce ‘buoy’ in North America.
One matches the British pronunciation, which also happens to match the pronunciation of ‘boy’: " bɔɪ "

The alternative American only pronunciation is marked phonetically as " ˈbuːi "

clear explanation!

Thank you very much!

I’m happy to have helped - and welcome to the forums.