why I need to change back to atmega 328P setting again since I am using atmega 328 ?
Because the IDE does not know about the 328, and the bootloader will be telling it that the chip is a 328P to make up for that. This is "OK" because the 328 and 328P are nearly identical, and certainly identical as far as Arduino is concerned. So the only time you need anything to know about the 328 is when you are burning the bootloader, because in that case the software is reading the chip type directly off of the chip. (thereafter, the IDE reads the chip type from the bootloader, and the bootloader lies.)