Ah, I see what you are doing, you are subtracting 10% from the 12.38% payout rate and thing that the difference is the rate of return.
That is incorrect. You need to use the IRR formula. The rate of return would be 4.49%.
If the payout rate is 12.38% then on a $100,000 deposit you would receive $1,032 per month ($100,000 * 12.38%/12). The internal rate of return of a $100,000 cash outflow followed by 120 $1,032 monthly cash inflows would be 0.367% per month [0.367% = RATE(120,1032,-100000)]
0.367% per month would be an annualized rate of 4.49%
[4.49% = (1+0.367%)^12-1]
So the return is 4.49%, not 2.38%.