How can last trade be within NBBO

Hi

What could cause the last trade to be within the Nation Best Bid/Offer, for example:

National Best Bid 30 / Ask 30.10

How could the last trade be 30.05 within the spread without the NBBO changing?

If someone placed a limit bid at 30.05 the NBBO would step up.
If someone placed a market buy they would get filled at the Ask.

I know market makers will fill within the NBBO but this is often within fractions of a penny.

So under what circumstances could trades happen within the NBBO without it chaning?


Thanks, any recommended reading on this topic would also be appreciated.

 

nbbo is 30 at 30.05.
30.05 gets lifted
offer moves to 30.1: new nbbo is 30 at 30.1

 

That's one possibility. It might take split second. Another possibility is a hidden order being hit. Someone placed hidden order at 30.05 and either buyer, trying to buy at 30.1 hit it on same venue or seller, selling at 30 hit it. Third -some sort of dark pool just posted the trade.all imo from my limited, non pro experience
And I'm pretty sure it's not all possibilities

 

Also, a bank thats axed could make a market to a customer to trade at mid.

The bank will cross the shares with the market not even moving.

 

Depends which venue, but could certainly be midpoint match orders going though.

 

bottom line-under current messed up micro structure possibilities are endless. and that's applies to everything. trades at bid,trades at ask,mid point..Thanks SEC!