Level might be slightly on the piss. Do you have a big flat plat you could rest it on?
Otherwise might be the feet/wheels on the cabinet and/or their fasteners?
The best way to check a level is to use it to draw a level line on a wall then soon the level 180 degree and hold it against the line. If the level is accurate it will line up with the mark. If the level needs replacing it won't.
Edit @nefarious beat me to it.
@Bobbo started
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Level might be slightly on the piss. Do you have a big flat plat you could rest it on?
Otherwise might be the feet/wheels on the cabinet and/or their fasteners?