Yours measures larger due to manufacturing tolerance and the coiling of the pipe, it is 3/4' copper, but will never go into that fitting, which in my opinion is labelled very incorrectly. 3/4' pipe that we all have in our houses is 7/8', or 0.875.
Go pick up a 1' steel pipe at lowes and measure it, it is much larger in OD than 1' and the ID depends on the schedule (i.e. As explained, pipe sizing is often not the OD or ID, it is nominal. As suspected, that fitting is for 3/4' OD tubing, which 3/4' copper is not.
