Here's a great example of what I was talking about with VIRT and their algo.
They stack under the ask as shown in 1st snippet below:
Then, if the the asks above them get hit they immediately disappear from the ask (because they don't actually want to sell any shares) as shown in 2nd snippet below.
Once buying slows or enough MM's stack above them, they immediately reappear to help stack, but ensure they have enough MM's/padding above, so they don't get hit. Shown in 3rd snippet below. Same algo every time and regardless of whay anyone says, what they are doing is NOT "making a market." It's an attempt to control the ask.