How do I interpret an RGB LED max amperage given as a range e.g. 30mA-70mA?
Without seeing the datasheet, it's not possible to confidently give a single reason, but here are a few possibilities:


Each color within the RGB package almost certainly have different \$V_f\$ and \$I\$ ratings. The rating you are looking at may just be trying to encompass these values, as you have already assumed.
It's possible that the current ratings are based on duty cycle. Many LED datasheets will provide a constant-current value (e.g. 20mA) as well as various ratings based on lower duty cycles (e.g. 50mA at 50%).


You'll need to find the datasheet or ask the manufacturer to be sure.