[Shader] Progress shader acts differently on certain devices

Apology, if this has been posted before.

I have this shader which does something like a progress bar.

Weirdly on some devices it gives the opposite effect of what i wanted. Below is a screenshot on how its expected to behave and how it looks like on a HTC m7

THANKS!

 Shader "Custom/ProgressBar"
 {
     Properties
     {
         _MainTex ("Empty (RGB)", 2D) = "white" {}
         _FullTex ("Full (RGB)", 2D) = "white" {}
         _GradientTex ("Gradient (RGB)", 2D) = "white" {}
         _progress ("Progress", Range(-0.1, 1.0)) = 0.5
     }
     SubShader
     {
         Tags { "Queue" = "Transparent" }
         Pass
         {
             Blend SrcAlpha OneMinusSrcAlpha
 
             CGPROGRAM
             #pragma vertex vert_img
             #pragma fragment frag
 
             #include "UnityCG.cginc"
 
             uniform sampler2D _MainTex;
             uniform sampler2D _FullTex;
             uniform sampler2D _GradientTex;
             uniform float _progress;
 
             float4 frag(v2f_img i) : COLOR
             {
                 half4 empty = tex2D(_MainTex, i.uv);
                 half4 full = tex2D(_FullTex, i.uv);
                 half4 gradient = tex2D(_GradientTex, i.uv);
                 return lerp(empty, full, step(gradient.x, _progress));
             }
             ENDCG
         }
     }
     FallBack "Diffuse"
 }