Flexibility vs Mobility
The best way to think this one through is to imagine “flexibility” as your passive range of motion; it is how far you are able to bend forward . This is usually limited to tonicity of the muscles, tendons, ligaments, fascia, scar tissue and joint capsules.
Mobility on the other hand is how you function within that range of motion. Just because you have good passive range of motion, it does not mean that you are capable of functioning optimally within that range.
Always build mobility as well as flexibility.