The "Inmarsat arcs" are where the last known location of the airplane is (the last "ping") – or in other words, the aircraft's last ping was made from near those arcs. The arcs do not represent the flightpath of the airplane
If we assume that the crash site is near the last ping (an assumption that can be wrong, as an airplane can glide for quite some distance…), and if we assume that the last radar contact is correct (which is not necessarily the case…), then we can draw many many different flightpaths from the last radar contact to the last ping.
Here are four purely hypothetical flightpaths out of many many more hypothetically possible flightpaths:
There are more likely flightpaths (e.g. the first one), and there are less likely flightpaths (e.g. the other three) – it would be good to have more information. For example: from what locations were the other pings made from? Missing such crucial information, the search area might be way way off…
There are many more things that are wrong in what people write and say about the MH370 flight, but to find that out is left as an exercise for the so inclined critical* reader.
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* critical, from Greek "kritikos", “able to discern”.
