When you attach your volume to an instance, AWS provides a message saying:
Note: Newer Linux kernels may rename your devices to /dev/xvdf through /dev/xvdp internally, even when the device name entered here (and shown in the details) is /dev/sdf through /dev/sdp.
As shown in the screenshot below:
This isn't really a bug. You can run the following command on your putty terminal to see the volume name of the EBS volume that you attached to your instance:
it will display something as follows:
Where /dev/xvdf is my volume, which, as per the first screenshot, started as /dev/sda