Static noise margins are exact voltage differences, not timing guarantees. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
High margin subtracts input high from output high
The high output is 4 volts and the high input limit is 3 volts, leaving 1 volt of margin.
4V−3V=1V
Low margin subtracts output low from input low
The low input limit is 2 volts and the low output is 1 volt, leaving 1 volt.