maximum soil resistance

Hello everyone,

I have set the soil model in the following picture.

In the result, When the longitudinal displacement of pipe is greater than yield displacement (592.25 mm), I expect to see 573.3 N/m of soil load in longitudinal axis. 

However, the result shows 656 N/m. I'm not sure what model wrong. In additon, the horizontal will happen the same situation when the pipe displacement is greater than yield displacement. (I have not captured for horizontal case)

I notice that my most most models happens the same situation. The maximum soil resistance is a bit greater than what I set in the soil input.

Thank in advance