"Stroke selection" doesn't restrict the output to the selection, the selection border is used as a location for the stroke, which in effect straddles that border.
At that point a quick and dirty solution is to just hit [delete] which will discard the part that is inside the selection, but this will create a tiny gap between the text and the outline.
A better solution is to move your new layer under the text, so only the part that is outside the text will show (the rest will be hidden by the text).
For better results, stroking a path is cleaner than stroking the selection.
What you want to do is perhaps:
Text to path (more accurate path that doing alpha to selection followed by
- Add new layer
- Remove the selection
- At that point your outline straddles the text border
- For an outline that just extends the text, move the outline layer under text layer (in that case you have to use a line width that is twice as big as what your want, since half or it will be hidden).
Otherwise you can use the