The amount is correct — it's the gross amount we send to the EU. Perhaps it would have been simpler to just put the net amount — £250 million — which is still a huge amount. Even if they did that, though, people would claim they were promising to spend the entire £250 million on the NHS, which would not be correct either.

Whatever the pledge was, people would have nitpicked and claimed it was a lie. There's no way to succinctly provide enough details on the side of a bus to satisfy everyone.