Don’t confuse high name recognition with popularity
Above is a chart based on the final polls in the 2012 London Mayoral race when Johnson just squeezed to victory with a margin of three points. As can be seen none of the pollsters had the gap so close.
Overstatement can happen when one of the people being the subject of the survey has very high name recognition.
So treat any Boris polling numbers with a degree of scepticism.