This is a sciency thing about precision. The classic example is the old joke of “this dinosaur skeleton is ten million years and two weeks old - they told me it was ten million years old when I started, and I’ve been working here two weeks”, the point being that the age was roughly ten million years, not tell million years exact to the day.
Scientists talk about “significant figures”, the number of digits that are actually important and not just extra zeroes, e.g. 10000 has one (probably!) and 0.0103 has 3. So when someone says it’s 70°F they are probably rounding to two significant figures, so for the conversion you should do the same, i.e. 21°C. If they said it’s 70.0°F then you’d convert to 21.1°C.
This is a sciency thing about precision. The classic example is the old joke of “this dinosaur skeleton is ten million years and two weeks old - they told me it was ten million years old when I started, and I’ve been working here two weeks”, the point being that the age was roughly ten million years, not tell million years exact to the day.
Scientists talk about “significant figures”, the number of digits that are actually important and not just extra zeroes, e.g. 10000 has one (probably!) and 0.0103 has 3. So when someone says it’s 70°F they are probably rounding to two significant figures, so for the conversion you should do the same, i.e. 21°C. If they said it’s 70.0°F then you’d convert to 21.1°C.