Keep in mind AMR value is not DPS. Tuned correctly, it can be close, but its just calculations based on stat weights which are dynamic. Generally, even though there is the potential "glitch" (if you want to call it that), going with the thunderforged piece in the example will still be a direct upgrade for you stat for stat. It's just a weird occurrence that makes it appear less valuable after equipping (which just means weights changed after equipping the piece.)