i just set up my system to keep the usb ports powered during standby. seems to work for most devices except for iphone.
at first i thought it wasn't charging the phone because the screen never came on as it usually does when plugged into a wall charger or plugged into the same USB port but when the computer is powered up.
then i realized it was charging, after i had left it plugged in for a while and battery showed being fully charged.
Unless your a computer engineer there's no way for you to know what the standby mode is and how it's different from regular mode. I'm not one either but I do accept the fact that there can be differences in the different modes and even though I don't understand it I accept it if it works and as they say if it ain't broke don't fix it.
I have found usb ports to be different from one another and the front being different from the rear in certian circumstances. Maybe you can try different usb ports to see if they all do the same thing.