Except that top-up funding from the high needs block is calculated on the assumption that the school has provided the first 6K and so the LA is only responsible for funding above 6K.
If there were statemented children in the school when the calculation was made or children who were unstatemented but had assessed high level needs, it would be clear that the school would need to provide the first 6K for those DC.
However, although there may be a legal obligation to provide the first 6K for all children, it is assumed that children on SA or SA + don't need anywhere near the first 6K funding and so 5K is sufficient to meet the needs of all the DC on the SEN register.
If DC did really need an extra 6K, then they would, of course, be recognised (by the school/LA) as having high level needs. Ergo, only DC with recognised high level needs ever receive the first 6K.
There is therefore the financial incentive to schools to minimise need and want to use the devolved SEN budget to meet the needs of all DC on the register rather than spend the whole budget on one child whose needs are likely to add up to, but not exceed, 6K.