Hi Jan,
It is possible that George was transported, served his sentence and then returned back to the UK (not many convicts did, but it was possible). George could then have committed another crime and be sentenced again.
I don't think there is any way of telling whether this was your George Boyce, but on 28th June 1869 George Boyce was committed to 7 years imprisonment after being convicted of 'simple larceny following 2 previous convictions' at Worcester Court