I suppose jno's point is that the precise wording of the statement was
"I have some sympathy for the motives behind the Charlie Hebdo attacks"
This is a rather weaker statement than "hav[ing] sympathy for the Charlie Hebdo terrorists". If one believes that the motive (or part of it) was revenge against a 'deeply offensive' depiction of Muhammed, then having some sympathy isn't too surprising, and doesn't mean that the attacks are implicitly being condoned by 27% of British Muslims -- merely that, perhaps, they can understand some of the motivation.
Actually they don't understand it, I don't think, because almost certainly the offensive cartoons were merely an excuse for a terrorist attack. The figure in that lot that ought to be focused on is the 11% who argued that Charlie Hebdo "deserved" to be attacked. That is an attitude that genuinely is disturbing, although again one shouldn't deduce that 11% of Muslims would then be happy to carry out such attacks. Loads of the time people, rightly or wrongly, argue that such and such deserves only the most horrible fates, but a massive proportion of those people would never do anything about it personally.
There is much to be done to try to combat these attitudes, but we need to be careful to oversensationalise the figures, or read too much into them either way.