> A column group is a set of columns anchored at a slot (groupx, 0) with a particular width such that the column group covers all the slots with coordinates (x, y) where groupx ≤ x < groupx+width and 0 ≤ y < yheight. Column groups correspond to colgroup elements. Not every column is necessarily in a column group.
What is the sentence
"Not every column is necessarily in a column group."
meant to convey as important information to the reader?
It seems to me that a column can only not be in a column group if it is an implied column. Implied columns only occur when there are no colgroup elements at all, meaning that *no* column is in a column group in this case, or after the last defined column group in a table (when there happen to be required more column slots in a row than there are columns explicitly defined).
I suggest rewording the sentence to
"Implied columns are not part of any column group."
as that seems to make it much clearer what you actually want to state.