This is nothing to worry about. It is normal for returns lodged in the same batch, on the same day to be processed at different times. This is due to the way the ATO processes returns.
Each return is grouped for processing based on common items contained in the returns or characteristics of the return. For example, returns containing a high level of work related expenses or that generate a very large refund generally take longer to process as they are subject to a higher level of checking by the ATO.
Having dealings with other government departments also delays returns. For example, being the recipient of Family Tax Benefits usually adds about 5 days to the processing time of a tax return, as the ATO needs to communicate with the Family Assistance Office before they can finalise the return.