It's because it's using the default setting of Dutch inflection notation:
Alterations are made with different names: add -is for sharp, and -es for flat (these are Dutch note names, other languages are available). LilyPond figures out where to put accidentals.
The -is and -es are for their relative phonological stresses.
If you want to use another suffix you can change the language setting. For example:
This way if you used English you would mark -s and -f for sharps and flats respectively.