From the linked page, about half of that £400bn is equity in state-owned oil companies with no stake in oil fields outside the north sea; as someone who lives in a country that had an albatross state-owned oil company, that's going to become a very interesting investment once those oil fields dry up in the next 20 years.
The article itself says the rest is partly because of lower taxes on the oil industry, but mostly because
1. The peaks of oil production in the UK's and Norway's fields were at different times when the market price of oil was different.
2. Norway has fewer fields but more reserves which made extraction more efficient.
It's also worth pointing out that the £6bn/year figure in the Independent article seems to be a bit of a wank too.
The article itself says the rest is partly because of lower taxes on the oil industry, but mostly because
1. The peaks of oil production in the UK's and Norway's fields were at different times when the market price of oil was different.
2. Norway has fewer fields but more reserves which made extraction more efficient.
It's also worth pointing out that the £6bn/year figure in the Independent article seems to be a bit of a wank too.