According to press reports it seems a strong possibility that certain estates have not been claiming this extra inheritance tax allowance which applies when residential property is left to the children (including step-children and adopted children) and to grandchildren or other direct descendants. The provisions governing its application are complex. Possibly those administering estates themselves without expert assistance are missing out.
Also bear in mind that the amount of the allowance has increased in relation to death after 6th April 2018 from £100,000. It is now £125,000 so the total inheritance tax allowance on the death of the survivor of the couple could now be up to £900,000.