Verified answer

That happens usually when you have other programs running in the background that take longer (or don't) shut down. Like the other answerer said, Just choose later and manually restart after you are sure all other stuff is shut down.

it depend on programs you are installing, some of them just ignore restart command, anyway sometimes you need to install more software at the time so if you restart your machine manually after you are done it is only two clicks more at the end and you can be sure that you finished what you started

i just say later and restart manualy anyway