Robe of the Whistling Fists + Celestial Fists

[fischsemmel]

Question about haste stacking.

This thread got me to go look at the buff slot #s for different haste spells/songs/clickies on the wiki and on allakhazam. Enchanter haste is slot 1, jonthan's whistling warsong (bard song, the clicky on ROTLC) is slot 2 (but all other bard hastes are slot 1). The haste on the epic is slot 5. The wiki and zam don't disagree with each other on any of this.

With other buffs, like strength or a resist, if that slot number is different, the effects stack. But not with haste?

Is the wiki/zam wrong about what slot these different haste effects are in, or is there some extra mechanic in there that stops haste from stacking when other effects in the same slots would stack?