At least, when you get the +10% from the untekked item, you get it to all the % there was on it.
Just imagine how awful it would to get two or three +10% from a same weapon if all of them were increased separetely on each tekking.
On a side note, it's also quite annoying when you're trying to get that +10% out of a regular weapon, and when it finally does, the special ability you get from it is utter shit, while you had one that was one or two levels higher on previous tries.