A maximum payload may be specified to be less than tested MTOW minus empty weight in some cases, as a precaution against conditions that may reduce MTOW -- which depends on strip elevation, barometer, and temperature.
The thinner (higher and hotter) the air, the less a given amount of power can lift, in the case of a quad-copter or similar aircraft, or the longer it will take to reach takeoff speed and the slower climb will be, in the case of a conventional winged drone (electric or otherwise, though at least electric motors don't lose power with altitude). Responsible drone manufacturers may (and should) specify a maximum payload spec that will keep gross weight below the least MTOW for foreseeable conditions.
There may also be cases where, for one reason or another, payload is based on testing conditions different from your flight configuration -- for instance, if you're using a heavier battery pack than the one used for testing, you'll have less margin below MTOW than the test condition. Payload limits may also be specified below MTOW minus empty weight for reasons of center of mass location, payload restraint limitations, or other engineering reasons.