Canadian constitutional law has something called the 'living tree doctrine', which states that the constitution can grow and evolve over time, being reinterpreted in new contexts. To push the metaphor a bit further, I'd point out that a living tree is considerably more resilient than a dead tree, and is likely to adapt to conditions which might otherwise destroy it.
I don't think anyone can seriously claim that the commerce clause of the US constitution was intended to grant the vast powers which it has been used to uphold; but because the US constitution is -- theoretically -- not subject to growth and reinterpretation the way that the Canadian or British constitutions are, a legal fiction has been adopted instead.
If the commerce clause had been interpreted within the context of the Canadian or British constitutions, it would probably have been handled as "we're going to read one new power into this" on a number of occasions, rather than the "yes, this clause gives you the power to do everything" which seems to have occurred in the US.