Do I understand Ohm's law correctly?
Going by the information you provided, it appears that the 12V resistance applies to the ABS material: 12V/1.0 Ω = 12 amps and 12V/1.2 Ω = 10 amps which is exactly as quoted.  For the 24v material, assumed to be PLA, the currents calculate out to 24/3 = 8 amps and 24/3.4 = 7.06 amps. This is a small discrepancy from the quoted 8 and 9 amps. However, there is no need to doubt Ohm's law. In many cases of this nature, the quoted voltages and currents are meant to be nominal and often won't exactly conform to Ohm's law. You should consult the data sheet for the product to see under what conditions these numbers are specified.