Brand new Grout repair dried darker than existing brand new grout
Few things are perfect in this world, and that includes grout. Some variables:


Dye lot: Factory mixes can vary by batch due to variations in color compounds
Water content: A substantial difference is the water quantity between the original grout and yours could affect color
Water contamination: Do you have iron or other minerals in your water? Was the bucket clean?
Age: It's possible that a longer cure time will result in a lighter color