The key you search is the worst case.
When searching a key near to the border, you have only one half of tame and wild that overlap.
In that case you need ~22 times more DP than expected, this can happen
If you count the total number of jumps:
Each time you restart from scratch a program (or a client), you have the overhead of nbKangaroo*2^dpbit, this is the average time that each kangqroo produce a DP.
This is why you need an nbKangaroo*2^dpbit the lowest possible compare to sqrt(N).
Tame: [0.......................N]
Wild: [-N/2+k..............N/2+k]
k is the key to search
To improve the search on the border it is possible to have a non uniform sampling and increasing kangaroo density near the border, it compensates a bit.