The graphs aren't always a great measurement on how the sound will be perceived by the human ear.
It shows the K701/2 as having as much bass as the HD650's and M50's. Of course, the human ear won't perceive it as having nearly as much bass compared to those 2.
As for the difference in sound between older versions and newer versions of the M50, I don't think that there really is any. It's possible that some people have compared the newer version to the older version without burning them in, which is why they may seem a tad lighter on the bass when compared to an older version which has had many hours of use.
As somebody else said, the drivers are identical and should produce the same sound signature. Different ear pads may effect the sound very slightly, but I think the ear pads are also identical.