Oh there are. But we can not say if all the 13% reported earn minimum wage or not. I'm not saying there isn't. There just isn't 13% that's for sure. Even if it is at 12.9% we just really can't know.
Sure, but then you are also missing all the people who make below minimum wage because they are not formally employed. Which is a large number of people in Mexico. For example, the salaries of domestic workers are hardly ever reported (the average is around 2 minimum wages, but it is unclear what the lower bound is and is not like they work only 40 hours in practice, see: [1]), and the earnings of street food vendors probably vary a lot (it might often be above minimum, but in plenty of cases it could also be below).
But by the same token, people are going to drift in and out of minimum-wage work, either through unemployment or better employment. It's not like it's the same fixed 13% of the population we're talking about. So it could well be a greater proportion of workers who at one point or another have worked for minimum wage.
[1] http://www.jornada.unam.mx/2014/02/05/sociedad/039n1soc