Are you referencing foreign on foreign grading? As you've discovered, some countries can hardly grade their own correctly, much less another. So your question of left up to buyer is appropriate. Although, I do take issue with any coin that category, it is too wide and vague. If you look at our NGC registry of foreign coin values, the Sheldon scale is used, meaning MS equals Uncirculated while XF and AU are 59 and under the magic number of 60. Buyer must then decide from pix, is it live or is it memorex!
John1, then according to grading protocols, both sides NUMERICAL grade is added and averaged out. Hence an XF-45 obverse paired with an MS-62 reverse would average would be an AU-53, and since the extreme difference in sides, would most likely get a "details" tag for whatever the reason the obverse got such a grade, as "harshly cleaned" or "Damaged".
If it was just one coin we are talking about I could see it possibly being XF-45 on the obverse and MS-62 on the reverse. But the OP is saying he is seeing this designation for multiple coins and in different countries. So I think they all can't be XF obverse and MS reverse.