This is actually the expected behavior. When calculating the curve between two points, their outer neighbors' values are taken into account. Let us take a set of 4 points P0, P1, P2 and P3. The curve between P1 and P2 is influenced by P0 and P3 in the following manner. The straight line defined by P0 and P1 would be the tangent at P1 and the straight line between P2 and P3 would be the tangent at P2.
This is better shown in your second image -- for the second pair of [6, 6] you have marked, the values of 11 and 6 influence the curve between the inner points - both at 6.. As the right outer point is again 6, the curve tends to go horizontal at the second "6" point.
You can find more details in this blog post
by Charles Petzold and in his demo
the Telerik team