In the guide for applicants there is written:
(GfA ITN 2015)
The calculations in the application form assume that 50% of recruited ESRs on ITNs will be eligible for the family allowance. According to the Commission, normally only ~15% of recruited ESRs are ultimately eligible for the family allowance and, therefore, they have not yet had a scenario where the maximum grant amount has had to be inflated to cover this, though they said they would of course consider doing so if it were to be the case for a project.
Presumably this suggests that most ITN projects end up coming in slightly under the maximum grant amount as not all of the family allowance will be claimed.