The following bug has been logged on the website:
Bug reference:      13972
Logged by:          Jacob Zneider
Email address:      zn(at)dbml(dot)dk
PostgreSQL version: 9.4.5
Operating system:   OS x 10.11.3
Description:        
Try the following:
select * from json_to_record('{"aA":1,"bB":[1,2,3],"c":"bar"}') as x(aA int,
bB text, c text)
first two columns will be empty.
select * from json_to_record('{"aa":1,"bb":[1,2,3],"c":"bar"}') as x(aA int,
bB text, c text)
All columns are mapped. 
select * from json_to_record('{"aa":1,"bb":[1,2,3],"c":"bar"}') as x(aa int,
bb text, c text)
All columns are mapped.