After all the reports about the Google Pixel and Google Pixel XL devices failing, Google announced that they have made some changes to the manufacturing process to make sure that audio issue would not repeat itself.
So when the people say the words RevB on the boxes of the device, they though that Google made some hardware changes to the new Google Pixel and Pixel XL and believe that the newer device was worth buying since it comes with better hardware but that was not the case at all.
Google later explain that the Rev. B on the box was an identifier for the packaging and not the device. They added that they did not make any revision to the hardware of the device but did dos some minor improvement to the manufacturing process.
That means the new Pixel and Pixel XL is slightly better now but only in the sense that the microphone will not be breaking down that easily but the rest of the device should work the same.