Question about parameter in safety stock method-BS

Hi Guys,

I am testing  the parameter in BS recently.

This is what I find,

1.

service level 98%

target days' supply      5           7           10           15

safety stock                198        147       83           0

Which means when the service level is 98%, target days goes up, then safety stock is down.

When I set target days' supply to 15, safety stock is 0. 

2.

target days' supply  7

service level       99%    98%  99%

safety stock       317      147    36

Which means when the target days'supply is 7,   service level goes down, then safety stock goes down.

Could someone give me any idea about these 2 cases?  why this happen?

Thanks a lot

Michael