Apparently, something is causing Siri to have 2 different voices. A good voice and a bad voice.
The bad voice is very robotic and you can't understand what is being said. The good voice is very fluid and speaks well. It's very difficult to understand Siri with the robotic voice. I really would like to know why it changed. They say it's and iOS bug, but I think it's a software update, saving battery power by rendering the voice in a different way.
Have you experienced this? Have you hears the difference? Isn't the robotic voice annoying?