a is a float; 0.7 is a value of type double.
The comparison between the two requires a conversion. The compiler will convert the float value to a double value ... and the value resulting from converting a float to a double is not the same as the value resulting from the compiler converting a string of text (the source code) to a double.
But don't ever compare floating point values (float, double, or long double) with ==.
You might like to read "What Every Programmer Should Know About Floating-Point Arithmetic".
