Whether the plan gets you there, and how the interest underneath it is actually calculated. These project forward through contributions, employer matching and growth, and show the answer in today money rather than a headline figure that inflation has quietly hollowed out.
The single most important thing a retirement projection can tell you is the inflation-adjusted answer. A projection showing a large future balance means very little on its own, because prices will have moved too. What matters is what that balance buys in today terms.
The savings account calculator models how a bank account genuinely pays: daily accrual, bonus rate periods, and the conditions attached to them. That is deliberately different from an investment projection, because a savings rate is a known quantity while an investment return is an assumption.
The last three work the interest itself rather than what it grows into. Simple interest is charged on the original amount only; compound interest is charged on the balance including interest already added, which is why it accelerates and why the gap between them widens dramatically over long periods.
How often it compounds matters too. Daily compounding beats monthly, which beats annual, even at an identical quoted rate. That is exactly what APY exists to capture: it converts a nominal rate into the effective annual figure, so two accounts advertising different rates and different compounding can finally be compared honestly. In the UK the same idea is called AER.