Return on this page compares two balances. There is no contribution in between. Gain is the ending amount minus the starting amount. Total change is that gain divided by the start.
Monthly rate = (ending ÷ starting) ^ (1 ÷ months) − 1. Annual rate = (ending ÷ starting) ^ (12 ÷ months) − 1.
From 10,000 to 15,000 in 24 months, the gain is 5,000, the change is 50%, the equivalent monthly rate is 1.70% and the effective annual rate is 22.47%. The annual rate is not the monthly rate times twelve.