Question about the Sprite mask implemented with the Stencil buffer

Hi. am looking for someone who is familiar with the shaders. So, there is a simple example of what I'm asking about. I have a Plane with the next shader

 Shader "Custom/World Fog Of War" {
     Properties{
         _Color("Main Color", Color) = (1,1,1,1)
         _MainTex("Base (RGB) Trans (A)", 2D) = "white" {}
     }
 
         SubShader{
         Tags{ "Queue" = "Geometry+3" "IgnoreProjector" = "True" "RenderType" = "Transparent" }
         LOD 200
         ZWrite off
 
         Stencil{
         Ref 1
         Comp notequal
         Pass replace
     }
 
         CGPROGRAM
 #pragma surface surf Lambert alpha:fade
 
         sampler2D _MainTex;
     fixed4 _Color;
 
     struct Input {
         float2 uv_MainTex;
     };
 
     void surf(Input IN, inout SurfaceOutput o) {
         fixed4 c = tex2D(_MainTex, IN.uv_MainTex) * _Color;
         o.Albedo = c.rgb;
         o.Alpha = c.a;
     }
     ENDCG
     }
 
         Fallback "Legacy Shaders/Transparent/VertexLit"
 }

And also a Sprite with this shader

 Shader "Sprites/Occluder1"
 {
     Properties
     {
         [PerRendererData] _MainTex("Sprite Texture", 2D) = "black" {}
         _Color("Tint", Color) = (1, 1, 1, 1)
         [MaterialToggle] PixelSnap("Pixel snap", Float) = 0
         _AlphaCutoff("Alpha Cutoff", Range(0.01, 1.0)) = 0.1
     }
         SubShader
     {
         Tags
     {
         "Queue" = "Geometry+2"
         "IgnoreProjector" = "True"
         "RenderType" = "TransparentCutout"
         "PreviewType" = "Plane"
         "CanUseSpriteAtlas" = "True"
     }
         Cull Off
         Lighting Off
         ZWrite Off
         Blend One OneMinusSrcAlpha
 
         Pass
     {
         Stencil
     {
         Ref 1
         Comp notequal
         Pass Incrsat
     }
 
         CGPROGRAM
 #pragma vertex vert
 #pragma fragment frag
 #pragma multi_compile _ PIXELSNAP_ON
 #include "UnityCG.cginc"
 
     struct appdata_t
     {
         float4 vertex   : POSITION;
         float4 color    : COLOR;
         float2 texcoord : TEXCOORD0;
     };
 
     struct v2f
     {
         float4 vertex   : SV_POSITION;
         fixed4 color : COLOR;
         half2 texcoord  : TEXCOORD0;
     };
 
     fixed4 _Color;
     fixed _AlphaCutoff;
 
     v2f vert(appdata_t IN)
     {
         v2f OUT;
         OUT.vertex = mul(UNITY_MATRIX_MVP, IN.vertex);
         OUT.texcoord = IN.texcoord;
         OUT.color = IN.color * _Color;
 #ifdef PIXELSNAP_ON
         OUT.vertex = UnityPixelSnap(OUT.vertex);
 #endif
 
         return OUT;
     }
 
     sampler2D _MainTex;
     sampler2D _AlphaTex;
 
     fixed4 frag(v2f IN) : SV_Target
     {
         fixed4 c = tex2D(_MainTex, IN.texcoord) * IN.color;
     c.rgb *= c.a;
 
     // here we discard pixels below our _AlphaCutoff so the stencil buffer only gets written to
     // where there are actual pixels returned. If the occluders are all tight meshes (such as solid rectangles)
     // this is not necessary and a non-transparent shader would be a better fit.
     clip(_AlphaCutoff - c.a);
 
     return c;
     }
         ENDCG
     }
     }
 }

This is a Sprite picture - a black ring with a transparency inside and outside it. And the result is looking like that

The questions are:

1. How can I remove this transparent area outside the sprite?

2. Why this happens?

Thanks for the replies.