If one sets up a field to validate as 'member of value list'
AND
If that value list is custom and it includes a dash separator
THEN
... typing a dash in the field will PASS validation. 
It seems that FileMaker gave a single hyphen a special role as separator but forgot to handle it from validation context. Please see attached file. Clear the drop-down on text1 and type a dash in it. I envision some User wanting to select 3.75 (the line between two values) and can't so they type dash in the field. It's the only character outside of legitimate values that is accepted. I went back and checked version 11 and same behavior so this has been here for a long time.
Part of me wants to say that the dash might come in handy - as an 'all' or 'none' entry but taking advantage of unexpected behavior/bugs can be risky. If used, it should be clearly documented but I wouldn't recommend it. In all, knowledge is power which is why I'm posting about it just so others are aware of it. I will report it as a bug as well.