O gauge is 1:48 and HO was defined as Half O. Thus HO should really be 1:96 (double the 48). Is there a reason why HO was scaled as 1:87? Motors too large to fit smaller engine shells, could not cast fine details at 1:96, track width derived from European trains metric sizes drove the HO 1:87 decision (this last would be my best guess), or
Have not read of an explanation but sure there is a reason for the 1:87 scale.