A 4 stroke engine fires one cylinder every 720 degrees of crankshaft rotation. So your Merc 140 only supplies 2 pulses per turn. Depending on the ignition and engine, some ignition systems will fire twice, the 2nd time is wasted as the cylinder is on its exhaust stroke. Do you have any access to test equipment that can verify your dash tach accuracy? Most timing lights with advance have tachs built in, handy for checking advance rate, total advance, and initial timing. Sounds like you have a good ear for engines to notice the tach is off, all this time that tiger thought it had a V-8.
