Hearts and arrows refer to the optical symmetry of a diamond whereas the GIA symmetry grade refers to physical symmetry. While poor physical symmetry will have a detrimental impact on the hearts and arrows patterning, there are many parameters that can impact the symmetry grade without having a significant impact on hearts and arrows.
Here’s a quote from this GIA article written by Ron H. Geurts:
“For example, if nine of the parameters are within the Excellent range but the table is off-center by 0.7%, the best possible symmetry grade is Very Good. If all 10 parameters are within the Excellent range, the expected symmetry grade would be Excellent. But consider a round brilliant that is out of round by 0.7%, with crown angle variation of 1.1° and girdle thickness variation of 1.1%. Even though all three parameters are within the limits for Excellent, the combination of these three symmetry features (and any others found on the diamond) may result in either an Excellent or a Very Good symmetry grade, depending on the visual assessment.”
This means it is very possible that a diamond with superior optical symmetry can receive only a very good symmetry grade from GIA and vice versa. For example, one way a GIA can knock an H&A is due to poor meet-point symmetry, referring to how well the corners of the facets in a diamond come to a point.
For a more in-depth discussion on diamond symmetry, please read my article on symmetry and polish.