The term commonly used for "holding period return" is "cumulative return".
The issue is that you're comparing two different formulas for return and mistaking them for interchangeable values.
Yield to maturity (YTM) is annualized (it's also forward looking).
Cumulative return is a historical measure of how much money an investment has earned for an investor during a historical time period and is not annualized. Annualized performance shows what the average annual return was for this historical period.
- Cumulative Performance formula = (Initial Purchase + Gains) / (Initial Purchase)-1
- Annualized Performance formula = ((Initial Purchase + Gains) / (Initial Purchase)) ^ (1/N)-1
(where "N" is the number of years.)
- Cumulative performance for your scenario is 20%
- Annualized performance for your scenario is 10%