You can read about APFill CMYK here
Q: Why results in CMYK in Photoshop and results in APFill differnt although in RGB they are the same?
A: Photoshop convert RGB to CMYK with self development algorithm. So algorithms in Photoshop and for example in Corel Draw are different. APFill uses formal convertion algorithm. If you want get average CMYK like in your editor, you can take average RGB color in APFill and convert it to CMYK in your graphic editor.
Q: I set RGB color in Photoshop and fill area with it, then I measure color density with APfill. But average RGB in APFill results and RGB color in Photoshop are diffrernt.
A: The fact is that Photoshop and Corel Draw convert colors according to Monitor Color Profile. If you remove Monitor Color profile then results in Photoshop and in APFill will the same.