one more question about Meursault & Sancerre ?

This is a tough question to answer.

Since there are so many wineries within the region and each winery usually has more than one product to offer. It is hard to draw a conclusion saying that ALL wines from Meursault or Sancerre are 100% Chardonnary or Sauvignon Blanc. Even though we know that white wine seldom uses more than one grape varietal for each product. But I will not draw a concluded statement saying they are 100% , coz it really depends on wine makers' style. They can mix in a very small percentage of other grape varietal to bring out the character of each wine.

Not exactly, coz wine makers will adjust the flavor by mixing in a small quantity of other grapes, so it may not be exactly 100%.