Why can two different airlines have different travel durations for the same itinerary?

The differing durations are a result of the aircraft that are used to fly the route by the two airlines. Delta uses a Boeing 757 aircraft while the Air Frances uses the slightly faster Boeing 747 and Boeing 777 aircraft on this route.

Depending on plane ETOPS, route can be different, even if the planes have the same engine number. 777 has quite a big ETOPS...

edit: To add some explanation. ETOPS (Extended range Twin Operations) is a rule which allows 2 engine planes to fly over oceans. Depending on how strongly the manufacturers promise that their engines are reliable the plane may be certified to fly routes where they are upto 120, 180, 330 minutes from the nearest airport.

A flight from the USA to Europe wouldn't normally be very much longer even for 120min ETOPS.

While this is just as much of an educated guess as all of the other answers, I would suspect that part of it is probably due to congestion at one or both terminals at the scheduled times of day. You can see this effect even on domestic flights. Even for the same cities on the same airline with the exact same model of aircraft, it's not unusual for scheduled durations even on domestic routes to vary by 5-15 minutes depending on time of day, especially at major hubs (which both Boston and CDG are.)

I do suspect that aircraft type also has something to do with it in this particular case, though. As other answers have mentioned, the 757 cruises a bit slower than the 777 or 747 (at 35,000 ft: Mach 0.80 for 757 vs. Mach 0.84 for 777 or 747-400.)

Edit: My answer originally stated that the 757 was limited to 120 minute ETOPS (e.g. that it was required to stay within 120 minutes of a potential diversionary field.) However, according to Boeing, 757-200s (both RR and P&W engine configurations) were upgraded to 180 Minute ETOPS certification. I have updated the map below from gcmap to show areas where the 757 is allowed to fly under 180-minute ETOPS, which includes the entire North Atlantic. As such, the 757-200 is not limited on where it can fly between BOS and CDG by ETOPS regulations, so ETOPS shouldn't be a factor in why the Delta flight takes longer.

Different airlines will estimate different times based on their own data. As such, it is reasonable to expect a few minutes variation from one airline to the next.

Large variations between flights at significantly different times of the day can also occur (even for the same airline). This is often due to the congestion of planes at peak times of day. I recall an article (that I can't find at the moment) that noted one airline had more than an hour of extra time for a particular route that arrived at a very busy airport. The same route earlier in the day didn't need the extra buffer.

For the example you illustrated, the flights nearly overlap. However, the Delta flight leaves at 7:00 (a peak traffic time). It could easily be that the Delta flight is anticipating sitting in "heavy traffic" waiting to take off. Since departure times are based on the time leaving the gate and entering the queue to take off, this causes a longer flight.

Likewise, 7:00 AM might be a higher traffic time than 8:00 AM for the arrival airport.

I have observed within Europe, that some airlines tend to overestimate the duration that their flights take, for flights of 2:30 duration of around 10-12 minutes. This way, even if there's unexpected delay, the plane will still arrive on time.

A pilot once told me that this has behavioral reasons. People don't mind being a bit late, but being a bit later than expected, that's the worst!

Whether Delta does that, and to what extend they do this with long-distance flights, I am unaware of.