I have too agree with lamarsy and wtp that it probably are a double IF problem as you only hear one transmission and not two that mixes.
When that WS1010 are set to receive 121.5MHz it could use an oscillator at 132.2MHz to mix with the frequency to produce a 10,7MHz signal that more easily can be amplified and filtered.
But that oscillator will also produce a 10,7MHz signal from a 142.9MHz signal and if the band filter for VHF air in the scanner isn't attenuating enough of a 142MHz signal it will be received and heard. The VHF air filter goes up to 137MHz and are probably passing 142MHz with very little attenuation. Modern scanners, and more costly old ones, use tripple IF system that also uses a first IF of something like 250MHz to avoid this double IF issue.
And as G7RUX explained a FM signal will be heard more loudly in AM mode if it isn't exactly on the frequency, like 142.920 that you have in your bell fleet system.
/Ubbe