Tallman has it figured out. The MFJ frequency display is a conventional "EPUT", or Events Per Unit Time counter.
Most all frequency counters fall into this category. It will only show a steady, accurate reading if the carrier remains above a minimum level. If you simply turned down the steady carrier power, you would reach a threshold level where the count begins to fluctuate randomly. Might be a half Watt, might be more, might be less. The negative modulation peaks on the 'scope are clearly very near the zero-amplitude level.
During those intervals, the counter has no input. Causes a reading that will always be below the actual frequency of your carrier.
Some counters will maintain a steady reading with no more than 10% or 20% modulation, and the reading fluctuates randomly until you stop talking. Other counters are sensitive enough to hold steady up to 60% or 70% modulation.
But the 100% level on the 'scope guarantees that the carrier's amplitude will fall below the counter's sensitivity limit a random percent of the time. Results in a randomly-low reading.
Until you shut up.
A trick that was used on older tube-type radios was to install a counter jack on the back that tapped some carrier energy from the radio's driver tube. In a tube-type radio, the driver is never modulated and allows a steady reading whether you're talking or not. Nice feature of this trick if you used it on a Browning transmitter was that the counter would give you a reading with the Spot button pressed, no need to key the mike.
This doesn't work on solid-state transmitters, since both the driver and final are modulated. Tubes are just different.
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