That's what I'm not fully understanding at the moment. Supposing those were sell orders rather than buys, what you mentioned, if they sent 5K at $13.41 and there were loads of bids around that price, I would expect a filled price around that quote but if I got a fill at $13.15 instead, I'd like to see what could be the cause to try and minimise it in the future.
Those bids might be taken by other traders who've sent their orders at the same time split second before you or those bids might have been withdrawn by the time your order was filled. So even though you see the bids, they could be gone by the time your order is filled.
How to minimise it in the future? Use limit orders!!
Nicely done.