something odd in my silhouette overlay shader

here is my code below:
----------------------------------------------
Pass
{
Stencil
{
Ref[_MaterialID]
Comp Always
Pass Replace
}
Cull Back
ZWrite On
ZTest LEqual
CGPROGRAM
#include "UnityCG.cginc"
#pragma vertex vert
#pragma fragment frag
sampler2D _MainTex;
float4 _Color;
struct vertexInput
{
float4 vertexOSITION;
};
struct vertexOutput
{
float4 pos:SV_POSITION;
};
vertexOutput vert(vertexInput input)
{
vertexOutput output;

output.pos = mul(UNITY_MATRIX_MVP, input.vertex);

return output;
}
float4 frag(vertexOutput input):COLOR
{
return _Color;
}
ENDCG
}
Pass
{
Stencil
{
Ref[_MaterialID]
Comp NotEqual
}
ZWrite On
ZTest Greater

CGPROGRAM
#include "UnityCG.cginc"
#pragma vertex vert
#pragma fragment frag
float4 _OverlayColor;
struct vertexInput
{
float4 vertexOSITION;
};
float4 vert(vertexInput input):SV_POSITION
{
return mul(UNITY_MATRIX_MVP, input.vertex);
}
struct fragOutput
{
float4 col:COLOR;
};
fragOutput frag(void)
{
fragOutput o;
o.col = _OverlayColor;
return o;
}
ENDCG
}
----------------------------------------------

here everything works fine to me
but, when i add another Empty Pass at the Top, odd things happened.
the farther object's overlay sillhouette just be replaced by the nearer object

i guss the neaer's 2nd Pass(have added another Empty Pass, so it's 2nd now) happened after the farther's 3rd Pass, as for ZTest Pass, the color replaced

 

any help would be nice