The sine function is equal to one at these three values of $\displaystyle\theta\usepackage{cmbright}\Huge\$ , but they are not the only values on this interval for which the function equals one. $\displaystyle\theta=\frac{\pi}{{2}}\usepackage{cmbright}\Huge\$ is also in this interval and $\displaystyle{\sin{\theta}}={1}\usepackage{cmbright}\Huge\$ at that location.

Remember that the graph of sine has a hill/valley pattern that repeats forever in both directions. Each hill and valley pattern together is $\displaystyle{2}\pi\usepackage{cmbright}\Huge\$ units wide. Because of this, values of the function will repeat with regularity.