I'm not sure why it's doing that. When I place an image and select it (the frame) with the Selection tool, it always reads as 100%. If I resize it (by Command or Shift-Command dragging a handle), I can see the percentage change as I'm changing it, but the display always reverts back to 100% when I release the mouse button. I can only see the changed percentage if I look at the content with the direct select tool, but the frame is always at 100%.
If I know that the image needs to be at 100%, I can set it on the contents and then fit the frame to the contents. Would that help?