Sixty degrees is an exact figure, but almost nobody reads it exactly. The sextile is granted an orb, a tolerance either side of the exact angle, and the size of that tolerance is one of the places where astrological schools quietly disagree. Traditional practice assigned orbs to the planets rather than to the aspects, so a sextile from the Sun carried a wider working range than a sextile from Mercury, simply because the Sun was held to have a larger sphere of influence. Most modern practice reversed this and assigns the orb to the aspect instead, which produces a narrower and more uniform tolerance for the sextile than for the conjunction or the opposition. Neither convention is derived from anything measurable, and a chart read under one can show a sextile that the other does not.
There is a second and older distinction. In sign-based aspect doctrine, which governs Hellenistic and much traditional practice, two planets are in sextile when they occupy signs sixty degrees apart, whatever their exact degrees. A planet at the first degree of Aries and a planet at the twenty ninth degree of Gemini are in sextile by sign and nowhere near it by degree. Degree-based reading, which is what most contemporary software draws, would show no aspect at all. The sextile suffers more from this than the square or the trine because its reputation is already marginal.
That reputation is the third thing worth knowing. Ancient sources treat the sextile as the weaker relative of the trine, effective but requiring something from the person before it does anything. The distinction survives in the modern habit of calling it an aspect of opportunity rather than of gift.