In Navigraph, it is giving the actual magnetic heading between the two VORs. Skyvector is giving the VOR radials. They are not the same thing.
It is important to understand that the heading shown for a Victor or Jet airway on a low or high altitude chart is not (necessarily) the actual compass couse you fly between the two VORs.
The VOR radial is what you set into the course selector (OBS) of your nav indicator in order to fly the depicted radial path over the ground with a centered CDI needle.
VOR radials are referenced in degrees clockwise from the “zero” degree radial of that specific VOR
The VOR’s zero degree radial is aligned to magnetic north at the time the VOR is first put into service. Once that is done, the alignment of the zero degree “north” radial is almost never changed again. However, as time goes by, the actual location of the magnetic North Pole changes. In the western US, the change is quite significant from year-to-year.
Both MMM and MLF VORs had their magnetic north zero degree radial set in the year 1965, at which time the magnetic variation was 16 degrees east. Now, in the year 2023, the actual variation at both VORs is 11 degrees east. In other words, the magnetic North Pole has drifted 5 degrees to the west of where it was in 1965 - which was 58 years ago.
As it happens, the majority of VORs in the western US are based on the magnetic variation in the year 1965.
Neither a VOR itself nor a Nav receiver “knows” (or cares) where the actual magnetic North Pole is actually located at a given time. If departing from MMM and flying to MLF on either the low altitude airway V21, or the high altitude airway J9-107, as long as you set your Nav receiver course selector to 16 degrees, and keep the CDI needle centered, the aircraft will track the ground path of those airways perfectly. However your actual magnetic compass heading will be 21 degrees (assuming there is no wind).
In the year 1965, the actual magnetic compass heading would indeed have been 16 degrees, but not in the year 2023, because of the westward drift of the magnetic North Pole.