Why is $\displaystyle e_0^o \neq 21 + e_{21}^o$ ?

$\displaystyle e_x^o$ is complete expectation of life at age $\displaystyle x$

Using ELT15 (males) mortality table

$\displaystyle e_0^o = 73.413$ and $\displaystyle e_{21}^o = 53.497$

but I'm unable to conclude on that equation.