As per the man page of xmodmap:
Up to eight keysyms may be attached to a key, however the last four are not used in any major X server implementation. The first keysym is used when no modifier key is pressed in conjunction with this key, the second with Shift, the third when the Mode_switch key is used with this key and the fourth when both the Mode_switch and Shift keys are used.
I assume that keysym in position 5 to 8 (or in my case 12) are triggered by Mod1-5 and some other mysterious key, and therefore almost never used.