Shotwell / Photos attempts to avoid re-encoding exported images whenever possible. (Re-encoding JPEGs degrades their quality.) When "Quality" is greyed out that means Shotwell / Photos doesn't need to re-encode and so the Quality is moot, hence it's greyed out.
Shotwell / Photos attempts to avoid re-encoding exported images whenever possible. (Re-encoding JPEGs degrades their quality.) When "Quality" is greyed out that means Shotwell / Photos doesn't need to re-encode and so the Quality is moot, hence it's greyed out.