One side as more resistance or higher draw than the other.
If for example there is an 1157 bulb in one side and a 1034 in the other. Actually '73 may have 1157 bulbs from the factory but thats concept. A lamp that draws more current will make the falahser work quicker. Apply that to the wiring too. If a lamp sees lower voltage across it (poor ground, pooor feed, either, both) it won't draw as much current, flasher unit will work slower.
These are often a PIA to chase down so consider the journey a crazy puzzle.