It could be Google Flights is trying to combine multiple fares on a single ticket which are not allowed to be combined and only checks the fare combination rules after you have gotten to the end of the fare quote. There's $300 seasonal fares for NYC-CDG and $400 for NYC-FCO. But these fares do not permit end-on-end fare combinations on a single ticket (you will need to book fares on separate tickets to get from ATL to NYC). This is different than the multi-city issues (which deals with combining multiple segments on a single fare).

SEASONALITY
OUTBOUND - PERMITTED 30SEP 19 THROUGH 12DEC 19 OR 24DEC 19 THROUGH 30MAR 20 OR 21OCT 20 THROUGH 15DEC 20 OR 05JAN 21 THROUGH 10MAR 21 FOR EACH TRANSATLANTIC SECTOR. INBOUND - PERMITTED 29OCT 19 THROUGH 24DEC 19 OR 08JAN 20 THROUGH 07APR 20 OR 01NOV 20 THROUGH 24DEC 20 OR 11JAN 21 THROUGH 17MAR 21 FOR EACH TRANSATLANTIC SECTOR.