What is the law in Texas for Sexual assault of a child?

I am so confused, and very appalled. I was looking at The Public Sex Offender Registration list online in the area where I live. (In Texas) I was shocked to see the time served for some of these offenders. Example: One male 25 yo was charged with Aggravated sexual assault of a 5 year old female victim and only served 48 months! Another charged with indecency with a child (sexual contact) only served 6 years. I don't understand, what happened to the 2007 Texas Legislature Handling of Sex Offenders? Isn't it said that Pedophiles can never be cured? Why are they walking free in our towns around our children after serving very little time in prison? Has another law been passed since then? What can the public to change this?