At http://eh.net/encyclopedia/article/short.retirement.history.us the statement is made that "By 1910, the average Confederate pension was only about one-third the amount awarded to the average Union veteran."

By Googling "Union Army Widows" additional information may be found, but it appears you have identified at least a part of the reason why there are to this day Confederate widows while little evidence exists to identify any living Union Army Widows.