If you change the aspect ratio of text by changing the Horizontal or Vertical size or percentage, the new sizes automatically become 100%. So the percentage means “percentage of current height or width”.
That makes sense for line graphic geometry that doesn’t have a “real” size or aspect ratio, but for text it means there’s no link to the original proportions (or point height) of the text. You can handle that with a written procedure “This font, this height, enlarge width to 137%”, etc., but it makes life easier if there’s still a record of the relationship of the current object to the default properties as inserted. AutoCAD and BricsCAD, for example, have a text width factor as part of the text “style”, so you can see how a given string relates to the default font shape.
I realize that means giving text a new scaling mode discrete from the sizing controls for general geometry, but I think it would make working with text simpler in some scenarios.