This is how the survey works: mothers (something like 10,000 of them, or getting on that way, which makes it a good survey) are asked among many other Qs, 'how was your baby fed at x weeks?' and if you, at that time, were giving breastmilk only, then you would say so, and you would go down in the survey as 'exclusively breastfeeding'. The survey is not interested in your history, so your baby could have had formula earlier for a short time, and he would still be 'ebf' for the purposes of the survey figures at x weeks.
I have not seen the current figures in detail but from previous knowledge, they do ask 'what age was your baby when he had other milk?' and you would put down '3 days' if your baby had formula at 3 days. So at that time, he was not excl bf, but he might be, age 10 days. Babies can go in and out of these categories, for the survey.