There is a good reason for the principle of only serving half a sentence.
A person gets to come out after half provided they have behaved well inside. Bad behaviour means the prison can add some of the time back on. Without this incentive then there would be much more rioting and trouble inside prison than there is - I'm not saying there is none but it could be worse.
Also when released early it's on licence. The sentence still exists until the end of the full term and so the person is subject to probation until the whole sentence is up and is therefore monitored.
None of the above is a comment on the individual case or the length of those sentences, I'm merely highlighting that there is a reason for the sentences being expressed in the way that they are.