Why is ISMR an approximation?

[Jammy]

indirectly standardised mortality rate is said to be an approximation of the directly standardised mortality rate.

I understand that this is because we use the standard age specific mortality rates to calculate the expected deaths in population being studied, instead of observed age specific mortality rates, assuming they are approximately equal.

(Expected deaths is the numerator in the denominator of the ISMR.)

Is this the reason ISMR is an approximation of DSMR? Are there any other reasons?

Thanks!

 

[Hemant Rupani]

I guess this is the only intuitive reason of indirect SMR being approximation of direct SMR.