Why is a terabyte not 1024 gigabytes here?

Because a terabyte is 1000 gigabytes, by the SI definition a drive manufacturer uses. The binary units have their own names, and a tebibyte is 1024 gibibytes. Both are in the list, which is the whole reason a 1TB drive appears to arrive with 931GB on it.

Are the conversion factors exact?

Yes, where an exact value exists. An inch is 25.4 millimetres and a pound is 0.45359237 kilograms by definition, not by measurement, so those carry no rounding. Every pair in every measurement round trips back to the original value, which is the check that catches a factor someone rounded.

Why is temperature handled separately?

Because it is a scale conversion rather than a ratio. Every other unit here converts by multiplying, since zero means the same thing in all of them. Temperature scales have different zero points, so twenty degrees Celsius is not twice ten, and treating it as a ratio gives answers that look plausible and are wrong.

Unit Converter

Seven measurements, exact factors, both directions.

Measuring

Amount

From

Seven measurements, exact factors, both directions.

How to use it

Choose what you are measuring. Length, mass, volume, area, speed, data or temperature.
Pick the two units. Anything that belongs to that measurement. Mixing them is refused by name.
Read the answer. Shown with both units written out, so it cannot be misread.

When you would use this

Unit conversion is the most searched arithmetic on the web and the tools for it are unusually poor, mostly because the factors get rounded somewhere and nobody checks. Every factor here is the defined value rather than a measured or truncated one. An inch is exactly 25.4 millimetres and a pound is exactly 0.45359237 kilograms, because the treaties that define them say so. That matters when you convert twice: a rounded factor drifts, and the second conversion does not return the number you started with. Temperature is kept apart from everything else because it is not the same kind of operation. The other measurements all convert by multiplying, since zero metres and zero feet are the same thing. Temperature scales have different zero points, so it is a scale conversion, and a tool that multiplies its way through gives plausible wrong answers. Data sizes offer both the decimal and the binary units under their proper names. A terabyte is 1000 gigabytes and a tebibyte is 1024 gibibytes, and conflating those two is why a new drive looks smaller than the box said.

Questions

Why is a terabyte not 1024 gigabytes here?: Because a terabyte is 1000 gigabytes, by the SI definition a drive manufacturer uses. The binary units have their own names, and a tebibyte is 1024 gibibytes. Both are in the list, which is the whole reason a 1TB drive appears to arrive with 931GB on it.
Are the conversion factors exact?: Yes, where an exact value exists. An inch is 25.4 millimetres and a pound is 0.45359237 kilograms by definition, not by measurement, so those carry no rounding. Every pair in every measurement round trips back to the original value, which is the check that catches a factor someone rounded.
Why is temperature handled separately?: Because it is a scale conversion rather than a ratio. Every other unit here converts by multiplying, since zero means the same thing in all of them. Temperature scales have different zero points, so twenty degrees Celsius is not twice ten, and treating it as a ratio gives answers that look plausible and are wrong.