Ah, true "active meter" PEP? Or passive-meter PEP?
A FT-101 will typically deliver a higher PEP in AM mode than in SSB mode.
The SSB peak output is limited by the ALC, a circuit that monitors for grid current in the final tubes. At the first hint of grid current, the transmit gain is reduced. IMHO an incredibly clever way of doing this.
But in AM mode, no such limit is in place. Turn it up and you get burned rubber in this mode. And a few other parts we came to nickname collectively "11-meter burn". Parts in both the driver and final stages bear the stress of excess drive.
But apparently the sound of the waveform you get is found pleasing by those who cherish overdriven AM transmit. There is a trimmer capacitor in the dedicated AM modulator circuit. It will influence the ratio of peak modulated power to carrier power. Normally it's set for max forward swing short of where it distorts the peaks.
This ratio, of the minimum carrier power needed to maintain full modulated peak power is all over the map. Wasn't a controlled parameter spelled out in the specs, so what any one radio will do is kinda random. Anyone who sees more than a dozen of the same radio model will know that the max performance each one can deliver will not be exactly the same from one specimen to the next. You might have to turn the carrier up to 75 Watts before the modulated peaks stop increasing alongside the carrier level. Or the peaks may increase no further past the 15-Watt carrier level.
The average we have found for most of the E radios was between 20 and 45 Watts. There will always be a threshold carrier level where the modulation peaks won't increase any further with increased carrier.
And there is a way to fudge this and improve the 'swing ratio' of this model, but that will have to wait for another day.
73