It might be that SDRSharp is sharp enough (pun intended?) to understand that tuning below 1.2 MHz for a center frequency is somewhat silly. If you tune below that you get strange spectrum artifacts like "negative frequencies" to the left of zero frequency. If it's set to only show 2 MHz of the 2.4 MHz actual bandwidth then going down to 1 MHz makes sense since "-200 kHz" would not be visible.

{^_^}