Your contributions and your growth are different numbers
A larger final balance does not mean you earned that entire amount. The contribution total is your starting balance plus the money you added. Growth is the difference between that total and the projected balance.
A simple example you can check
Start with $1,000 and add $100 at the end of each month. With 0% interest, you have $2,200 after one year: $1,000 + 12 × $100. At a 6% nominal annual rate compounded monthly, the projected balance is about $2,295.
How contributions are modeled
We turn the selected compounding frequency into an equivalent monthly growth rate: i = (1 + r/n)^(n/12) − 1. Here r is the annual rate as a decimal and n is the number of compounding periods per year. Each month, the balance grows by i, then the monthly deposit is added. This assumes deposits at month-end.
What the projection leaves out
The rate is held constant. Taxes, account fees, inflation, and changing investment returns are not included. For annual or quarterly compounding, the equivalent monthly rate is a modeling convention; actual account crediting rules may differ. Use this to compare scenarios, not as a forecast of guaranteed returns.