I understand that it sounds odd, but it's simply because there is a cut off point where households are defined as being in poverty. It's never a perfect calculation, but the Government, and organisations campaigning on behalf of children and families, all use a figure, so that they can see whether things are improving, or getting worse.
If there are 4 million+ children living in relative poverty, then there will be a spread of family incomes, with some children in deep poverty, and others only just below the poverty line.
School meals cost around £500 a year for one child, so it's not surprising that a £500 boost for a family pushed some people above the poverty line.