A:
They do mean different things.
As you said, abstentions would have the same effect as negative votes if the requirement is for "two-thirds vote of members present."
So, for example, if there were 18 present, you would need 12 affirmative votes to adopt a motion.
If 3 members abstain, you would need 12 of the 15 ballots cast to be affirmative.
If the rule is for a "two-thirds vote," the implication is for that proportion of those voting.
So, using the same example, if there were 18 present and 3 abstentions, you would need 10 of the 15 ballots cast to be affirmative.
See RONR pp. 397-98.