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Heh, I was about to explain that phenomenon too. It's so weird, because it's not as if English speakers can't say the flap [ɾ], they use it all the time in words like butter [bʌɾɚ] and ladder [læɾɚ], yet when they see the r written somewhere else like arigatou [aɾigatoː], they read the r like a retroflex English r [ɑɹigɑtoː] (not to mention they also usually don't pronounce the [a] correctly...)