You need to look at the thrust curves and other data on the NAR website. This will help clarify the difference between "thrust coming out of the motor at any given point in time" vs. the "total impulse delivered by the motor over the entire burn".
The letter class of the motor is only the POSSIBLE total impulse range *if* they packed the motor to the maximum allowed in that class. You can have a D motor that is *not* a "full" D (like the Estes D12). Look at the NAR data for various Aerotech D motors for a good comparison. Many are a full 20 Newton-seconds of total impulse, but some burn slower with a lower thrust level for a longer time, while others burn faster with a higher thrust level for a shorter time. That is why the number after the letter is important: it tells you the average thrust in Newtons. Divide that number by 4.45 to get the average thrust in pounds.
BUT you should also look at the curve, since many motors have large variation in thrust over time, and the initial peak thrust is also very important to get the rocket off the launch rod fast enough to fly stable. Compare many curves on many motors to see the wide variety available.
Heck, look at the not-so-subtle differences between the D12 and D11 and the C11 and E9 from Estes. They get the differences with different nozzle diameters and different counterbore depths.