I can't really say what the significance of the result is that you show without some more details on what exactly is being shown in the picture and where it is from.

Here though is a counter-example from the well-cited study of [Fich, Blitz & Stark (1989)][1] for our own Galaxy, which appears to show excellent agreement where the two techniques overlap between 5 and 9 kpc. In this picture the CO measurements are the solid bars and the 21cm measurements are the inner ones with dashed points.

![Rotation curve from Fich et al. (1989)][2]


  [1]: http://adsabs.harvard.edu/abs/1989ApJ...342..272F
  [2]: https://i.sstatic.net/QIfw5.png