In aperture priority the shutter speed will be calculated by the camera depending on what item you focus on. Focus on the sky, shutter speed will go high, focus on shadow, shutter speed will go low. The camera does not have a shutter speed determined (in aperture mode) until you focus and it determines shutter speed needed. Let up on the button, and again there is no shutter speed because camera doesn’t know the brightness of what you will next focus on.
In program mode, similar situation. No aperture or shutter speed is determined until the camera measures light from what you are focusing on -so it has no aperture or shutter info to show until you press button halfway. If you then let up on the button, it once again does not know what you will focus on, do shutter speed and aperture are not set.
I hope this answers you question. Incidentally the + and - buttons allow you to tell the camera to go brighter or darker than what it would normally calculate. This is if you find particular images too dark or too light. For example a bright shirt causing your subjects face to expose too dark, or a dark suit causing camera to overexpose subjects face.