Depth offset different on iOS vs Editor

I'm using a shaderlab pass with ZTest Greater to achieve a silhouette effect when an object goes behind something. In order to not make objects display silhouettes when their mesh self-occludes, I'm using a depth offset Offset 0,[_DepthOffset]. I adjusted the values for this offset on a per-object basis in the editor, but on an iOS device with the same camera setup the depth offset isn't large enough.

Is there some constant multiplier for depth offsets on iOS? What's the reason for the discrepancy?

There's nothing about it in the Unity Platform Differences doc.