photos on nexus phone (android) to mac - not all showing on screen when attempt

[happyfeet2015]

i am not sure what I am doing wrong. only certain (limited) photos show on my computer when i attempt to transfer them across? why is this?

when I have to tx photos from my Nexus to PC via cable I have to change the setting that comes up from "USB for charging" to "USB for file transfer" then the phone shows up as a drive with internal storage and I go to camera from there and see all my photos.