Issue with a blur shader

Hello Im pretty new to shader programming. So I tried to create a shader that blurrs everything behind a certain Object.
This version is just a simple implementation of a box blur with an offset.

Shader "Custom/Blur" {
Properties{
_Offset("Offset", Range(0,1)) = 0
}
SubShader{
GrabPass{
}

Pass{
Tags{ "Queue" = "Transparent" }

CGPROGRAM
#pragma vertex vert
#pragma fragment frag
#include "UnityCG.cginc"

struct v2f {
half4 pos : SV_POSITION;
half4 grabPos : TEXCOORD0;
};

sampler2D _GrabTexture;
half _Offset;

v2f vert(appdata_base v) {
v2f o;
o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
o.grabPos = ComputeGrabScreenPos(o.pos);
return o;
}

float4 gauss(sampler2D tex, half4 projCoord, float4 size)
{
float4 c = tex2Dproj(tex, projCoord + half4(-size.x, size.y,0.0,0.0)) +
tex2Dproj(tex, projCoord + half4(0, size.y, 0.0, 0.0)) +
tex2Dproj(tex, projCoord + half4(size.x, size.y, 0.0, 0.0)) +
tex2Dproj(tex, projCoord + half4(-size.x, 0, 0.0, 0.0)) +
tex2Dproj(tex, projCoord + half4(0, 0, 0.0, 0.0)) +
tex2Dproj(tex, projCoord + half4(size.x, 0, 0.0, 0.0)) +
tex2Dproj(tex, projCoord + half4(-size.x, -size.y, 0.0, 0.0)) +
tex2Dproj(tex, projCoord + half4(0, -size.y, 0.0, 0.0)) +
tex2Dproj(tex, projCoord + half4(size.x, -size.y, 0.0, 0.0));
return c / 9;
}

half4 frag(v2f i) : COLOR{
float4 col = gauss(_GrabTexture, UNITY_PROJ_COORD(i.grabPos), _Offset);
return col;
}
ENDCG
}
}
FallBack "Diffuse"
}

I want the boxblur to be always offset by the Offset value in pixels, but somehow it seems to depend on how far the camera is from the Object with the blur shader.

I think It has something to do with

UNITY_PROJ_COORD(i.grabPos)

or

o.grabPos = ComputeGrabScreenPos(o.pos);

I dont fully Understand what they are doing.

Make sure to set the render Queue above 2500 for it to work properly