The length of different internodes in a culm of sugarcane is variable because

Intercalary meristem is present away from apical meristem in primary permanent tissue. It is present at the base of internodes , e.g. in family - Gramineae of at the base of leaves , e.g. Piius or at the base of node , e.g. Mentha. Intercalary meristem is responsible for increase in lenght.
The shoot apical meristem present at the apisec of shoot , is self determining and autonomous organising centre. The primary growth and differentiation of primary tissues is entissues is entirely due to this meristem as it continuously divides giving rise to new cells . these are the apical meristems that increase the length of plant but not internodes variability.