The fence pictured here is indeed the same from a few photos back. However, it will be very helpful in describing proper grade management".
When setting any job, you want to have maximum ground coverage, which is the ability of the fence to follow closely to the grade. You also want to have a nice aesthetic flow to the top of the fence, meaning you do not want a fence jumping up and down from post to post. Now while there are times there is no avoiding constant change to the flow of the top of the fence or gaps in the bottom of the fence, more times than not these obstacles can be minimized.
In this picture here, you will see that the grade seems a bit "jumpy" (not flowing smoothly along the top. Part of this is, or can be, that the fence has aged a bit and that the posts have warped a tiny bit or settled a touch causing it to look "jumpy". This however, is not the reason for the dip towards the back.
This fence is set "to grade" (fence at all posts is to exact height intended). This is also the reason for the dip towards the rear. Had the 4th post (counting from the right of the picture back) been set about 2-4 below grade and the 5th post set about 2 above grade, this fence would have a much better flow to it and still maintain satisfactory grade contact. (The grade change numbers represent an approximation.)
If grade contact and flow are both important to you, the use of longer pickets in the areas where grade contact will be lost would be the solution.