Isn't the first year normally some of the lowest numbers?
Depends on supply constraints, which can kick in at any point in a life-cycle really.
Theoretically year 1 should be the period of highest demand, especially as a successor product, but with the least margin for error on supply-chain and manufacture.
The point being that its a good result, and shows good planning and general organization on Ninty's side. BUT you can't simply assume the sales per year are going to grow proportionately over time or be maintained at a high level over the long-term.
Again, this is not a knock on Nintendo, its just how the market works. Every product hits saturation point at some level, every product has a finite level of units manufacturable in a given year, so there are hard limits to whats possible.
That being said, desirability and demand is what allows for maximization within those hard limits.
For example; were Xbox more competitive would Playstation be holding up sales-wise so well ? Probably not. That failure is helping to uphold demand.