"The flip side of this is that if the fund earns larger than anticipated returns, in a DB plan the employer gets the surplus..."   That is not correct in many cases.  For example, if the company cannot show that since the pension plan began it was clearly stated to employees that any surplus belongs to the company, irrespective of whether the employees contributed to the plan or not.

As for actuaries doing their calculations properly, they cannot possibly predict with 100% accuracy where salaries, interest rates, investment returns, etc will go over the life of a pension plan.  That is why plans experience deficits or surpluses at different times including at wind-up or closeout of the plan.  There is far greater risk to an employer with a DB plan than a DC plan.