I tend to move up or down in 50Hz sections. However, the shortcut "Shift-F11" moves the TX frequency down 60Hz and "Shift-F12" moves the TX frequency up by 60Hz. On FT4 it's 90Hz.
There are probably many reasons why people might not be exactly so many hertz up or down from your offset frequency.
1. Their transceiver calibration is off (I would guess that not many people have carried out the FreqCal procedure and set the 'slope' and 'intercept' for their radio based upon checking against known frequency standards. See Section 13.1. Frequency Calibration in the WSJT-X User Guide)
2. They have either set an offset frequency manually, or they have gone up or down a few Hertz to avoid another station.
3. They've used some combination of the keyboard shortcuts, see "Keyboard shortcuts" under the Help menu in WSJT-X.