Back when the prohibited gun list was made in the early 90's, they prohibited by name anything that looked "scary" (i.e. anything military looking that had a pistol grip), including the Sig-550, MP-5, AK-47, FN FAL, etc. The reason the AR-15 was spared prohibition was because the DCRA made the case that since the C7 is our military's rifle, civilians need to be able to have access the AR-15 in order to be able to participate in service rifle matches. So the government only restricted the AR-15 and all variants instead of prohibiting. Since they named what they wanted prohibited, any new guns that came along afterward that wasn't a "variant" of those on the list is either non restricted or restricted, depending on barrel length. You may notice that the Swiss Arms PE-90 rifles are non restricted. This is because the guts of them are based off of the Sig-540 series of rifles,
not the Sig-550. Since the Sig 540 isn't listed as prohibited, they're legal.
However, lately the RCMP has taken to classifying according to the manufacturer's advertising. The GSG-5, for instance, was advertised as a copy of the MP-5, even though the gun does not have the same internals. The same was the case with the Sig-522.
In case you're curious what our overlords say is too dangerous for us common folk, here's the list:
http://www.rcmp-grc.gc.ca/cfp-pcaf/fs-fd/rp-eng.htm