Rufus developer here. This is because the save option is only for uncompressed VHD images, and uncompressed VHD images are just the same as DD images with an extra 512 byte footer added.
So, for all purposes, you can use DD to write the VHD image created by Rufus, and it will work just fine (with either the extra 512 byte footer being ignored as random data, if you use a larger sized disk, or being dropped, if using same exact same size disk).
Ergo, what Rufus creates when you click the save button is essentially a DD image since it can very much be used as such.
PS: For those who wonder why Rufus doesn't compress the VHD images it saves this is primarily due to the compression algorithm being used by Microsoft being very poor (IMO) and therefore not worth spending time implementing support for. This is also part of the reason why save as DD image is only available when clicking the advanced options.