I think some of you are a little confused, look at the definition of NPT. The taper is actually in the thread angle itself. Its not that the pipe (or hole) gets smaller on the end.The different thread angle is what causes the interference. This would agree with what john said above.
This is from the engineering tool box website.
the angle between the taper and the center axis of the pipe is 1o 47' 24'' (1.7899o)
truncation of roots and crests are flat
60o thread angle
pitch is measured in threads per inch