Interesting bid stacks

Looking at the bid stacks on PRL, I noticed a few of the lines have interesting bid stacks which I have not noticed before. I would normally expect these to be chronological, but they don't appear to be. ie in the image below for bids at 12, you see the top on was placed 10 June, the one from May, then some June again, and the dates bounce around a fair bit. I have never noticed this before and thought it seems a bit odd with later bids seeming to have priority over earlier ones.
Does anybody know how this happens?