OP: I’m trying to reproduce your interest and principal figures and am having difficulty.

FYI, here’s what I get using my trusty BA II:

N=60, PV=100, set to END

P/Y=C/Y=12 PMT = 2.2244 First year interest 11.16 principal = 84.47

P/Y=12, C/Y =1 PMT = 2,1935 First year interest 10.58 principal = 84,259

As rawraw pointed out, the outstanding principal will change with each payment, so in your monthly example, it is not correct to assume that the outstanding principal is $100 for the whole year when calculating the rate of return. As a matter of fact, the outstanding principal will always earn 12%.