In 2010, Canon had a 14.7% market share lead on Nikon. As of today, at worst, according to Hogan's data, Nikon has reduced that gap by 50% at worst, by 80% at best, and on average by 75%. Even if we take the worst case scenario, for any company to change a large market share differential by 50% is significant.
That is overly simplistic. Take a look at the worldwide DSLR market shares from 2006 to 2008:
In 2007 and 2008, Nikon nearly caught up with Canon. But Canon pulled far ahead again in 2010 and 2011.
In short, Canon and Nikon numbers fluctuate from year to year. But the Canon ~ 40% and Nikon ~ 35% numbers have been fairly stable since 2012.