Thats a good point - what's in the missing persons's report is often different than reality as people gain/lose and lie about their weight (guilty!), and families may not have seen the missing persons in weeks or months and weight can change quicker than you think (just ask me after the holidays). Height though is usually correct.
In the NamUs reports of an unidentified body the age/height/weight information comes from the medical examiner or coroner (not all coroners are doctors so it's usually from the ME) which was confirmed by the words "measured" in the report for height and weight of the unidentified report you referenced as possibly being Valerie. In cases where the remains are compromised through accident, the elements, decomp and/or scavengers, the NamUs report will say "estimate". The following is just an example from an actual NamUs file (not the whole file, just the parts showing height weight age but the case # is there is you want to look it up.

Unidentified Person / NamUs #UP12158
By bringing people, information, forensic science and technology together, NamUs helps resolve cases.
www.namus.gov

Date Body Found
January 7, 2014

Case Information
ME/C Case Number
2014-00205
Estimated Age Range (Years)
14-18
Height
5' 7"-5' 11"(67-71 inches) , Estimated
Weight
150-175 lbs, Estimated