And it's because they didn't hit level 70 on that version. As the game won't recognize it, just like with having all 6 classes at level 70. You will need to start a completely new one of these to get it to level 70 for achievement. This also holds true for the Hardcore character. While you may have 6 characters at level 70, the game doesn't recognize them as you didn't create them or get them all to level 70 on the X1 version.

The achievement for having 6 classes at level 70 also will not pop when you transfer over 6 characters that are 70. 360 version stopped receiving updates a long time ago, but the X1 version still is and they updated a lot of stuff. I also wasn't able to enchant or modify the 360 items, as they are a different version of the item and are not in the X1 version. So a lot of my level 70 set and legendaries were down right useless and I had to grind for the X1 versions. Or the X1 version added all new stats and bonuses that do not transfer or update the 360 items to the X1 versions. At least for me (there have been a number of updates since then so maybe some of this has changed), when I transferred over my 360 RoS characters, a lot of their items were corrupt or had error messages as an update completely changed the item. For me, I primarily was playing this Solo on the X1, as my friends didn't want to buy the game again. Though some achievements will be easier to get than others, such as crafting items, getting the mystic, get the 3 vendors to level 12, etc. Nothing will pop instantly upon loading up the game or a character. However, you will need to re-earn all the achievements. I did the transfer a little over a year ago and at that time, yes you can transfer the characters over from the 360 to the X1 and you can transfer them from either version (Normal D3 or RoS).

Diablo 3 export save pc to xbox one