"why would the jumper cables mean tapering amps?"
It has to do with the voltage. During bulk stage (also called "constant current" (cc) stage), the battery will accept all the amps the charger can supply. As the battery gets charged up, its voltage rises closer to the set point; in this case, 13.8v. Once at this point, cc ends and abs/"constant voltage" (cv) stage begins. Voltage holds steady at 13.8v and amps taper.
But the amps are also lower than what they would have been if the voltage was held at 14.5v. It would have taken longer for Vbatt to rise to the 14.5v setpoint, and all the while amps into the bank would be higher. So, at 13.8v, you're getting less amps to begin with, and then tapering from there. This increases charge times considerably.