Why were the Japanese attempts at peace during WWII considered unofficial by the US and official by Japan?

They were, from beginning to end, perfectly willing to stop fighting if the US would only allow them their gains in the western Pacific and on the Asian mainland. There never was a peace feeler in which they were actually willing to give in to defeat. Therefore, to summarize the Japanese position, it was "You're winning; we'll stop fighting as long as you let us win." It would be hard for anybody to take that kind of position seriously.

There was great division within the Japanese government, especially toward the end of the war. The diplomatic corp. wanted to negotiate and find peace, but the military leaders had all the real power and authority, and they were set on fighting to the last man. There were intercepted radio transmissions that said the diplomatic corp. wanted peace, but they didn't have the political leverage to actually follow through.

Also, Japan at that point had violated nearly every treaty and rule of engagement that existed (attacking pearl harbor before officially declaring war, torturing prisoners, attacking medics, suicide bombs, ect...) so even if they had tried to negotiate, it probably would have been seen as a trick.