Texture color discrepancy between block planes and cubes

In the attached screenshots using the ladder as an example, changing it to a 3d model but keeping the original texture will make the ladder noticeably darker. Also attached is a resource pack that changes the ladder block model but defers to the vanilla texture, so it is not a case of using the wrong texture. 

Note: screenshots were taken under the Night Vision effect to show that it is not caused by lighting. Attached extra screenshots from a dark area to further support this.