Actually, I made a test, and result is really weird!
I made a script that called once per second for 500 times /users/show with user_id ‘177254406’, and that print the number of followers. I tailed the output, and, as number of followers didn’t move, I went to the account and followed it myself, thinking that result won’t change as it was not correct (84086 instead of 84630) but it actually gained one follower (see in result file around try number 87).
To be sure it was due to my action, I then unfollow, follow again, then asked my coworkers to follow then unfollow the account, and their action were always reflected on the number of followers, even if it was wrong.
Also, in rare cases (3 over 500), I got the right number!
Please, see here the result file: https://jpst.it/Rxjb