query regarding after-close transactions

Curious whether anyone can tell me what's happening here:

On at least 3 occasions during the past 10 days transactions have been processed after the market closed to the effect that the closing price was each time pushed down by 0.5c

Here's today's closing trades:

Time SSN Price Quantity
16:10 589,477 21 40,714
16:10 589,476 21 9,286
16:10 589,474 21 16,009
15:57 566,822 21.5 50,000
15:57 566,821 21.5 50,000

So the price displayed as 21.5c at the 16:00 market close but then was pushed down to 21c by virtue of the transactions processed at 16:10.

As I said, I've seen this 3 times in recent days - how come?