Troll Lord Games

I was reading the adventure Assault on blacktooth ridge and every time I find a magic item, I stop and calculate the value with the aid of tab of pag89 of M&T.
I am one of those CKs that gives xp for magic items to players.
Since I am used to do things like a battering ram, I didn't noticed the pile of tabs about magic items with the xp value listed on them. Now I am aware of them, and a few question arises:
1)the value of xps of the items differs from the value in gp, why? I thought that the value in gp and xp were the same thing and the players always earn the whole value in gp but may earn less or more (depending on a lot of circumstances, for example charisma) of the value in gp
2)I have noticed that sometimes the value in xp is 1/10 of the value in gp, sometimes more and sometimes less. What are the rules behind that? How can I calculate the xp value of an item not on the list? I've got the tool to extrapolate the right gp value (the famous tab on pag89 of the MT) but I haven't any idea about the way of doing this with xp.

If someone of you knows an old thread of this forum where this matter is discussed, please tell me that.

I have tried to read through the lines of the many lists: the only thing I have understood is that the xp value of potions and scroll is a flat 200 less than the gold value. the rest of the values in xps of the magic items has got behind some calculations that only who have done that can clarify.