What causes the difference between Maine and Canadian lobsters?
First of all, Ramsay was primarily concerned that they were advertising Maine Lobsters and selling Nova Scotia Lobsters.  He actually didn't comment on the flavour.  

I've searched about 20 web pages (nothing authoritative) and the consensus is that the colder the water, the better the lobster.  

But, the waters off of Maine are fed by the Labrador current, which is the same water that flows past New Brunswick, PEI and Nova Scotia.

If there is a temperature difference, it is likely not very significant, and any effect it would have on flavour is minimal at best.