I'm guessing you are probably using Data/Pkt mode in WSJT-X.
Many rigs have limited bandwidth. Some can adjust that bandwidth.
What rig do you have?
What are your bandwidth settings?
one reason may be that users who do not have CAT control of their rigs
are following our advice to use Tx audio offsets above 1500 Hz. This
ensures that any unwanted audio harmonics are greatly attenuated by the
rig's Tx IF filter. Of course they can elect to tune their VFO dial down
1 or 1.5 kHz if they wish to get to those lower frequencies.