Welcome, @kevindarmstadt, to the community!
There is a simple reason for this: the ULN2001 is a simple multichannel bipolar Darlington transistor, whereas ULN2002...2004 each have a series resistor at the base.
The ULN2001 is therefore driven like an npn transistor, except that it has two base-emitter junctions in series, as is usual with Darlington.
You will find the differences in fig 1 on page 3 of the data sheet.
Does it answer your question?
Regards
/Peter
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