To reproduce:
1. Open the attached project
2. Open scene MecanimSpeedBug
3. Seelct the "EscortVehicle01_REF" in scene, set it's "Movement Y" to 1
4. Open animator and open up the blend tree
5. Change the Pos Y from 2.5 to 1 and from 1.25 to 0.5
6. Enter play mode and look at the inspector of the escort vehicle. Under current speed it outputs the speed at which the transform moves. It shifts between three values a low one (~0.5), a middle one (~2.5) and a high one (~3.3). We are interested in the middle one as it's the average speed.
7. Set the Movement Y to 0.5 - the average speed changes from 2.5 to 1.25 accordingly
8. Set the Movement Y to 0.75 - the average speed changes to 1.66 but it should be 1.875

Explanation:
Root motion speed is not interpolated linearly in blendtree.
We will update the documentation on this.