A similar question was posted a while back and answered by another individual. My interpretation of that previous post is as follows:
If the filter you have is for a Twin Cam, the relief valve spring in the filter is of a higher pressure rating than the correct Sporty filter. Since your Sportster may very well maintain a lower oil pressure than a Twin Cam the oil could in turn bypass the filter all together resulting in unfiltered oil.
The Twin Cam oil filters are also of a finer filtering media. I believe the spec. is 10 micron as opposed to 25 micron as in the past.
Here's the skinny from the P&A catalog: The Super Premium (10 micron) oil filter is an upgrade for anymotorcycle currently using p/n 63796-77A or 63805-80A. It goes on to state this filter 63798-99 chrome, 63731-99 black will retrofit to most earlier models. If you cross reference this filter, it is interchangeable with late '84 and later XL models.
Give a look at page 722 of the 2002 P&A book for more specifics. From what the catalog states, you may be in luck.