The precise value, expressed in scientific notation, commonly referred to as exponential notation, is 1.84E-07.
0.0000001845 is equal to 1.845E-07. Unless you change the formatting of the cell to display more decimals, Excel will display a number that is very near to 0 as 0.
However, C# will retrieve the cell's actual value. When converting tiny values to strings, the ToString function uses the e-notation.
Alternatively, if you don't want to utilize the e-notation, you can specify a format string.