Even with all other factors being equal, unofficial round ratings have always been known to be off from one round to the next. In all honesty though, conditions for any given tourney day are never the same in the morning and afternoon. Warming temperatures, or a 5 MPH up/down tick or directional change in the wind can have an affect on scores.
I honestly wish that they didn't average the rounds when they make results official. Seeing that slight difference helps paint the snapshot of what went on that day.