The Retirement Security Report uses a scoring system designed to capture the key things that matter when building toward retirement income security. This means that there is usually more than just one thing to pay attention to, and all of the metrics can matter to the overall score for a retirement system. Sometimes, this means plans with identical or nearly identical provisions can score differently. 
A helpful example is to consider two defined contribution (DC) plans that have similar contribution rates, but score differently when it comes to retirement income security. 
The South Carolina Optional Retirement Plan and Michigan Public School Employees’ Retirement System have similar rules for contributions. In South Carolina, members contribute 9% of their pay and government employers provide 5% of salary contributions. In Michigan, members are enrolled paying 7% into their individual accounts, and employers match this with 7% too.
So both DC plans have 14% of member payroll total contributions as the default. 
And yet, when we developed RSR Benefit Scores for each system, the results came out different.
South Carolina SCRS DC Plan Teachers
Michigan MPSERS DC Plan Teachers
STW: 21 out of 25
STW: 14 out of 25
MTW: 19 out of 20
MTW: 13 out of 20
FCW: 25 out of 25
FCW: 25 out of 25
The main reason for these scoring differences is the rules in each plan about “vesting.” These are rules about when a DC plan member becomes eligible to claim contributions made by their employer. In South Carolina, members are immediately vested. If they leave after one year, two years, or 10 years, they get to take contributions made by their employer. 
In Michigan, though, members do not completely vest until they reach four (4) years of service. If they leave before then, they have to leave behind a portion of the 7% of payroll contributions their employer made. This might not seem like much, but it can be a big deal for Short-Term Workers—especially when it comes to their retirement income security — and that is reflected in the overall RSR Benefit Scores for each plan.
Details matter even within the same plan. In South Carolina, there are different assumptions provided by the retirement system for how salaries will grow for “General” employees (city workers and state agency employees) and how salaries will grow for “Teachers” (who are employed in K-12 public schools). This different set of salary growth assumptions means that the South Carolina DC plan for General members scores bit better than for Teacher members. (See this article for more details.)
In short, all of the provisions and rules for benefit plans matter. A common belief is that DC plans are better for Short-Term Workers than pension plans. And in general, that is true. But it does not follow that all DC plans work well for Short-Term Workers. The underlying metrics and plan design elements all matter.