The reason there is some variance in the watchdog time period is because there is going to be some variance in the current used to charge up the capacitor and some variance the voltage threshold that triggers a reset. If you look at page 3 of the datasheet you will see that there are two parameters called SWT Ramp Current and SWT Ramp Threshold. Both of these values have a min, max, and typical values.
If you divide Vramp(typ) by Iramp(typ) (1.235V/250nA) you get 4.94x10^6, which is the same constant you use in the equation to calculate the capacitor value needed for a specific watchdog time period. Now if you take the value Iramp(min) and Vramp(max), you will get a constant of 6.485x10^6. If you take the values Iramp(max) and Vramp(min), you will get a constant of 3.91x10^6. You can plug these constant values into the watchdog time period calulcation to find the maximum and minumum watchdog time periods for your system.