Horse armor's body, neck and ears (and possible other sections) are not offset from the horse entity itself. Other horse armor parts like head, mane, legs, etc. are not affected and are offset properly.

This is because the horse renderer's body, neck and ears override any provided dilation (0.1F).

Screenshots:

Code Analysis:

There is a 0.1F cube deformation added to the horse armor model layer, which can be see on the head and legs of the horse model. However, this deformation does not apply or is not obviously visible on the body, neck, and ears of a horse.

package net.minecraft.client.model.geom.ModelLayers

public static Map<ModelLayerLocation, LayerDefinition> createRoots() {
   Builder var0 = ImmutableMap.builder();
. . .
   var0.put(ModelLayers.HORSE_ARMOR, LayerDefinition.create(AbstractEquineModel.createBodyMesh(new CubeDeformation(0.1F)), 64, 64));
. . .
 }