Depending on the strain, a one-metre plant can produce between zero grams and several hundred grams, depending on how it's grown, how it's pruned, the talents of the person growing it, the strain of marijuana in particular. If you have a one-metre high plant and you forget to water it and it dies, your basic yield is zero.
It is arbitrary. I guess you have to start somewhere, but I find it a little humorous to put in that limit of one metre. I can imagine law enforcement going around with their tape measures, saying, “I'm sorry, you're one centimetre over.” Are they going to cut that centimetre off and seize it, or are they going to take the whole plant as a result? It is a little Kafkaesque, if you ask me, to have set it at that arbitrary level.