Of course, you've already realized that some of that is that smaller estates are more common than larger estates. But it seems unlikely that there are four times as many estates between $10 and $11 million as above that range.
People who expect to die with an estate subject to inheritance tax tend to prepare.
I don't know how common it is, but if the surviving member of a couple remarries, then the new spouse gets a separate exemption. And of course spouses inherit from spouses without tax. In theory this could last indefinitely. In practice, it is less likely. But if a married couple has $20 million, the first spouse could leave $15 million to the second and $5 million to other heirs. The second spouse could leave $10 million to a third spouse (after remarrying) and another $5 million to children with the first spouse. All without triggering the estate tax.
People can put some of their estate into a trust. This can allow the heirs to continue to control the money while not paying inheritance tax. Supposedly Ford (of Ford Motor Company) took that route.
Another common strategy is to give the maximum without gift tax each year. That's at least $14k per donor and recipient per year. So a married couple with two kids can transfer $56k per year. Plus $56k for the kids' spouses. And if there are four grandchildren, that's another $112k. Great-grandchildren count too. That's more than a million every five years. So given ten years to prepare, parents can transfer $2 million out of the estate and to the heirs without tax.
Consider the case of two wealthy siblings. They've each maxed out their gifts to their own heirs. So they agree to max out their gifts to their sibling's heirs. This effectively doubles the transfer amount without tax implication.
Also realize that they can pretransfer assets at the current market rate. So if a rich person has an asset that is currently undervalued, it may make sense to transfer it immediately as a gift. This will use up some of the estate exemption. But if you're going to transfer the asset eventually, you might as well do so when the value is optimal for your purpose.
These are just the easy things to do. If someone wants, they can do more complicated things that make it harder for the IRS to track value.
For example, the Bezos family invested in Amazon.com when Jeff Bezos was starting it. As a result, his company could survive capital losses that another company might not. The effect of this was to make him fabulously rich and his parents richer than they were. But he won't pay inheritance tax until his parents actually transfer the estate to him (and I believe they actually put it in a charitable trust). If his company had failed instead, he still would have been supported by the capital provided by his parents while it was open (e.g. his salary). But he wouldn't have paid inheritance tax on it.
There are other examples of the same pattern: Fred Smith of FedEx; Donald Trump; Bill Gates of Microsoft; etc. The prime value of the estate was not in its transfer, but in working together while alive or through a family trust. The child's company became much more valuable as a result of a parent's wealth. And in two of those examples, the child was so successful that the parent became richer as a result. So the parent's estate does count. Meanwhile, another company might fail, leaving the estate below the threshold despite a great deal of parental support.
And those aren't even fiddles. Those children started real companies and offered their parents real investment opportunities. A family that wants to do so can do a lot more with arrangements. Of course, the IRS may be looking for some of them.
The point being that the estate might be more than $11 million earlier, but the parents can find ways to reduce it below the inheritance tax exemption by the time that they die.