thanks, i checked that out and i'm only seeing 1 partition on this dev/sda1 ,
also noticed in the article you sent the link to that they are showing the same option choices while detecting linuxmint sylvia installed, so it seems to be a common situation. though they go on to say ;
If another operating system is present on the computer, the installer shows you an option to install Linux Mint alongside it. If you choose this option, the installer automatically resizes your existing operating system, makes room and installs Linux Mint beside it. A boot menu is set up to choose between the two operating systems each time you start your computer.
so i'm guessing that the screenshot is an oversight.
i'm reading in other sites about the problem but haven't seen anything that sounds like they know why it happens...so going to alternative installs via partitioning etc seems to be the option to create dual booting.