Why is child sex offender's sentence so lenient?

American mixed martial arts instructor Joshua Robinson was reported to have had unprotected sex with two 15-year-old girls, filmed the acts, shown an obscene clip to a third girl who was only six, and had in his possession 321 pornographic videos of children (Trainer jailed for sex with minors, making obscene videos; March 3).

Details of his other offences were not reported.

Given the reported facts, his sentence of four years' jail, with no caning, seems extremely lenient.

Robinson was no opportunistic offender. He had prowled the Internet for underage girls.

While I accept that no case is identical, the sentences meted out to other similar offenders seem far harsher in comparison to what Robinson was given.

For instance, Benjamin Sim Wei Liang was sentenced to 20½ years' jail and 24 strokes of the cane for offences of similar gravity (Man gets jail, cane for sexually abusing four underage girls; July 31, 2015).

Are there reasons not reported for Robinson's sentence?

Gan Boey Keow (Ms)