It's not odd it just means it hasn't be programmed into the acceptable answers. You should report it so it can be added. Duo knows that the courses are not perfect (with so many variables how would that be possible) but they have given us the: "Report a Problem" option. Or you can use the "Support" option (left side vertical) if you have left the page.
Edit I just noticed that "Είναι κάποια" was accepted for me and that was some time ago. Now, that is odd.