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I checked your form and you have assigned 1.28 as the calculation value for your options. 78 x 1.28 = 99.84 which is correctly being shown in the form. Since 100/78 is 1.28205~, rounding off to 1.28 causes the calculation to come short of 100.
If you change the calculation to use 0 decimal places, the value will be rounded to the nearest whole number: