From discussions with my main squeeze, Bruce Weigle of 1870 fame, think lobster in a pot full of water. Turn the heat up gradually, the lobster stays put until it's too late. Drop the lobster into a pot already boiling, and it goes bat shit crazy to escape. With smoothbore muskets the casualties build up gradually over time, so there is no immediate impact, so troops stick around and thus take more casualties. With Chassepot vs Zundnadelgewehr, significant casualties hit immediately over a short period of time, the troops immediately notice and often,  without orders, take measures to protect themselves by taking cover, backing off, going to ground. Thus, while the unit might become combat ineffective quicker, the overall casualty count drops. Now add in longer range and greater accuracy with rifles vs Brown Bess.

Similar for artillery.

VERY generic example:

a. Gravelotte, 301,332 on the field for both sides, 32,435 casualties total, or 10.7 %.

b. Borodino, 285,000 on the field for both sides (Bezonotzky, 2004), 72,000 casualties, or 25.3 %.

Exceptions, to be sure, but other examples are similar.