Cardboard SDK secretly changing FOV?

Im trying to draw a full screen quad in my Cardboard SDK project.

         MeshFilter filter = this.gameObject.AddComponent<MeshFilter>();
         MeshRenderer renderer = this.gameObject.AddComponent<MeshRenderer>();
         
         float distance = mainCamera.farClipPlane * distanceFactor;
 
         Vector3 p0 = backgroundCamera.ViewportToWorldPoint(new Vector3(0, 0, distance));
         Vector3 p1 = backgroundCamera.ViewportToWorldPoint(new Vector3(0, 1, distance));
         Vector3 p2 = backgroundCamera.ViewportToWorldPoint(new Vector3(1, 0, distance));
 
         float frustumHeight = Vector3.Distance(p0, p1);
         float frustumWidth = Vector3.Distance(p0, p2);
 
         filter.mesh = CreatePlaneMesh(frustumWidth, frustumHeight, distance);

Background camera is a normal camera to render distant elements (align with the head cameras) but this quad will be rendered by the GvrEyes. Now well if I render this with a background camera of 80 FOV it works in the editor and Galaxy S6 but it won't take the full screen in a Galaxy Note 4 unless I go to 120 FOV.

But, if I check the FOV of the gvreye cameras they say it is 80 at runtime in both phones and the editor! So it should work, certainly it is changing somewhere but I cannot find the real value.

This has allways been working for me in GearVR by the way.