See Using decimal, float, and real Data, and Data Type Precedence
In your second query,
776.2384 is a decimal number (and so are the two other values). So the calculation is done in that type, and the result is mathematically exact.
In the first, you introduce a
real. The other literals are promoted to that type, and the calculation is done according to IEEE 754 rules, in round up mode. IEEE 754 floating points are not exact, so you will generally not obtain the "mathematically exact" result you're expecting.
Quote from the articles above:
Because of the approximate nature of the float and real data types, do not use these data types when exact numeric behavior is required, such as in financial applications, in operations involving rounding, or in equality checks. Instead, use the integer, decimal, money, or smallmoney data types.