The crossing of two different cannabis strains form a new F1 (“first generation”) hybrid plant. However, all the Cannabis strains that exist in the market today are greatly heterozygous, meaning that any pollination (crossings) between such two different Cannabis strains will produce seeds with increased genetic instability.
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Thus, today each individual seed produced in a pollinated plant, even though it's essentially an F1 hybrid seed, is in principal different than all the other seeds in that same plant.

The fact that all seeds are different from each other in one particular plant means that the propagation of plants from such seeds, even though they originated from the same plant, produces plants with different DNA profiles.

That is why the only method accessible to Cannabis growers today, to maintain identical (“true to type”) offspring of one unique plant (termed a ‘mother plant’), is through cloning (either vegetative propagation or tissue culture).

In contrast, the conventional seeds industry terms an F1 Hybrid seed as a seed developed from the crossing of two distinct 100% homozygous strains – also referred to as “Parental Lines” (a ‘female plant’ and a ‘male plant’). The crossing of these two stable parental lines always produce the same F1 hybrid seeds, which are not only identical to each other, but are always 100% uniform in their DNA profile.