Unpublished GPS fixes on ILS approaches

With the Garmin 430W (and presumably the 530 as well) when one flies
certain approaches the Garmin has you fly to fixes that appear nowhere
on the published approach plate. For example, if you fly the ILS24
approach at ABE from STW (Stillwater) IAF, the Garmin will have you
fly from STW to D16.5 then 4 miles to MUDRE intersection. The fix
"D16.5" appears nowhere on the published approach plate. Is there any
discussion of this anywhere ? Why does the Garmin seem to need a fix
4 miles outside of another IAF ?

Thanks.