So you're clearly seeing signal where there shouldn't be any – this can have different reasons:
- Your transmitter actually transmits on frequencies it shouldn't
- Your receiver sees signal where there isn't actually signal
Both can again be caused by different things – the Veronica class of transmitters seem to be working with "kind of" two 180° offset intermediate frequencies of half the desired RF carrier. So your transmitter could actually just be leaking those on TX.
Also, your receiver might simply be picking up the strong signal although it's tune somewhere else completely, simply by the fact that the receiver's mixers never "calculate" the frequency difference you wish, but also other linear combinations of RF and IF – you can only counteract that by filtering, but filtering might simply not be sufficient if the input power is so large that things still make it through the filter in sufficient strength.
Then: every active component in both your receiver and your transmitter is potentially a mixer. Fundamental truth of RF design: every nonlinearity produces intermodulations (ie. sum and difference frequencies of the different signal frequencies present at the component). Most don't do that intentionally, and designers take care to eliminate the effects. However, especially amplifiers get the less linear the more you drive them with an overly strong signal – making them mix different frequencies present. So, suddenly, your receiver's (hypthetical, I don't know your receiver) IF amplifier becomes a mixer for the IF and the remnants of RF that passed through the "intentional" mixer, leading to surprising results.