nested sumproduct + offset

[BruceBayer]

I'm trying to understand how the offset() , n() , and offset() functions are used here to adjusted the sumproduct range. I've created a table below to show the spreadsheet. The formula in cell D3 is applied across the row
D3=+SUMPRODUCT(N(OFFSET($B$2:C2,,C1-COLUMN($B$2:C2)+2)),$C$6:D6)

E3=+SUMPRODUCT(N(OFFSET($B$2:D2,,D1-COLUMN($B$2:D2)+2)),$C$6:E6)

[TABLE="class: grid, width: 300"]
<tbody>[TR]
[TD][/TD]
[TD]A[/TD]
[TD]B[/TD]
[TD]C[/TD]
[TD]D[/TD]
[TD]E[/TD]
[TD]F[/TD]
[TD]G[/TD]
[TD]H[/TD]
[TD]I[/TD]
[TD]J[/TD]
[TD]K[/TD]
[TD]L[/TD]
[TD]M[/TD]
[TD]N[/TD]
[TD]O[/TD]
[/TR]
[TR]
[TD]1[/TD]
[TD]Period[/TD]
[TD][/TD]
[TD]1[/TD]
[TD]2[/TD]
[TD]3[/TD]
[TD]4[/TD]
[TD]5[/TD]
[TD]6[/TD]
[TD]7[/TD]
[TD]8[/TD]
[TD]9[/TD]
[TD]10[/TD]
[TD]11[/TD]
[TD]12[/TD]
[TD]13[/TD]
[/TR]
[TR]
[TD]2[/TD]
[TD]Invested[/TD]
[TD][/TD]
[TD]3[/TD]
[TD]4[/TD]
[TD]5[/TD]
[TD]0[/TD]
[TD]0[/TD]
[TD]0[/TD]
[TD]0[/TD]
[TD]0[/TD]
[TD]0[/TD]
[TD]0[/TD]
[TD]0[/TD]
[TD]0[/TD]
[TD]0[/TD]
[/TR]
[TR]
[TD]3[/TD]
[TD]Returns[/TD]
[TD][/TD]
[TD][/TD]
[TD]15[/TD]
[TD]35[/TD]
[TD]60[/TD]
[TD]60[/TD]
[TD]60[/TD]
[TD]60[/TD]
[TD]60[/TD]
[TD]60[/TD]
[TD]60[/TD]
[TD]60[/TD]
[TD]45[/TD]
[TD]25[/TD]
[/TR]
[TR]
[TD]4[/TD]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[/TR]
[TR]
[TD]5[/TD]
[TD]Period[/TD]
[TD][/TD]
[TD]1[/TD]
[TD]2[/TD]
[TD]3[/TD]
[TD]4[/TD]
[TD]5[/TD]
[TD]6[/TD]
[TD]7[/TD]
[TD]8[/TD]
[TD]9[/TD]
[TD]10[/TD]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[/TR]
[TR]
[TD]6[/TD]
[TD]Return per invested $[/TD]
[TD][/TD]
[TD]5[/TD]
[TD]5[/TD]
[TD]5[/TD]
[TD]5[/TD]
[TD]5[/TD]
[TD]5[/TD]
[TD]5[/TD]
[TD]5[/TD]
[TD]5[/TD]
[TD]5[/TD]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[/TR]
</tbody>[/TABLE]


Year 1, $3 invested, Year 2 $4 invested and Year 3 $5 invested. Each dollar buys a 10year annuity of $5 per year. So using the formula to aggregate the total payments over the life of the annuities.

Thanks for the help in understanding how the formula works.