My handwriting has always been pretty sloppy. My 9s come out like your 7s when I don't close the loop properly (I start at the bottom).
People confuse my lowercase r's for n's all the time too for a similar reason. Either I loop a little too much or I drag down the overhang so it basically is an n.
It never gets confused with 1, but in America, people were confusing it with 9 (!!), so I had to stop writing it like that. Can't please everybody...