The trades are sequentially numbered because they are at the end of the 16:00-16:10 single closing price auction. i.e. at a time somewhere between 16:10 and 16:11 the ASX says STOP! All overlapping trades are matched and then they all trade at a single final price.
Same thing happens on market open too.
Quite normal - have a look at a big stock like BHP or NAB on close as a comparison.
