The upper limit is limited by a surge protection device on the wiring panel, which is a 1.5KE20CA. The key limit is for constant dc voltages; it starts to become leaky as the voltage increases and reaches the range of its nominal surge clamp voltage of 20 V. It is possible to damage that component if the voltage is continuously held near its clamp voltage, causing it to dissipate a lot of power.
There are also other issues of internal heat generation, which could increase measurement errors at high temperatures—especially if powering sensors or peripherals from the datalogger.
This is why the limit of 16 V is quoted, which is within the normal operational voltage of lead-acid batteries. If the voltage creeps a little above this, especially when cold, it should not cause a problem.