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It’s because of iMessage, which doesn’t work like normal SMS messages, but instead uses your 3G/4G connection to receive messages. When you switched from Apple to Android, all of those iMessages you were getting from other iPhone users continue to get sent to your iMessage account, even though you can no longer access it with your Android device. To fix this, you either need to use your Apple Support profile to delete your old device, or, if you are still partly in the Apple universe and use a Mac, you can just go into your account settings in your Messenger application, and uncheck your old iPhone number so it is inactive.