The obvious answer is that when the bronze coinage was introduced the Mint faced many challenges involving short lived dies. Very many die pairings were tried out, without long term success. By 1863 much of this had been overcome when the best working die pairings had been put into effect, 6 + G for pennies, 7 + G for halfpennies and 3 + B for farthings.
That combination of die pairings then remained in effect until the next major changes in 1874.