C++ adding numbers from input file

You don't count the first thing you read, but you count the last thing twice. So you get the count right by accident, but of course get the sum wrong unless the first and last values happen to be the same.

You read something into num and then enter the loop. The first thing your loop does is read into num again, discarding the previous value. So the first value doesn't get counted. You then increment count and add it to sum whether or not the read succeeded, which counts the last value twice.

You want this:

myInput >> num;
while (myInput.good())
{
    // First count the value that we just read
    count++;
    sum += num;
    // Then read the next value
    myInput >> num;
}