In order to make sure it's a tab, you can click the «show / hide» button, that looks like a backwards «P» with a
double vertical bar (it's a
paragraph mark), and tabs will show up as an arrow pointing to the right.
To put it in simpler terms, if you hit the Return key (which
marks the end of a «
paragraph» in Word), even if your line spacing is set at 1.0, the before / after
paragraph spacing may make your text look as if it is
double - spaced: