Depends which years figures you use, doesn't it? What we can be sure of is that it snows on Mont Blanc, the snow compacts and is of variable thickness dependent on a number of factors and scientists use the more stupid definition of the height:
The height of a mountain with snow or ice on the summit is generally defined as its total elevation above a vertical datum (such as mean sea level) to the highest point of that snow and ice cap, though it can also be measured to the underlying bedrock.