1.84E-07 is the exact value, represented using scientific notation, also known as exponential notation.
1.845E-07 is the same as 0.0000001845. Excel will display a number very close to 0 as 0, unless you modify the formatting of the cell to display more decimals.
C# however will get the actual value from the cell. The ToString method use the e-notation when converting small numbers to a string.
You can specify a format string if you don't want to use the e-notation.