Why bch and btc has same address?

I did not send any BCH to BTC, nor BTC to BCH, but searching one address shows -

There are 2 blockchains with result(s) to your search

BTC address

BCH address

submitted by /u/xuapr2021
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source https://www.reddit.com/r/btc/comments/nnd3vu/why_bch_and_btc_has_same_address/