Gatt,
It's a combination of lag as mentioned above, and dispersion.
With the auto-coordination of buff guns, you can have 5 or more .50's shooting at you at once. Each stream has a dispersion effect, and has a convergence setting as well. The result is a pretty wide cone of fire that makes it relatively easy to hit.
Combine this with lag and the gunner can nail you in some situations that don't look right. Often, you start an evasive when he fires, but on his FE you are still flying straight for a second or two and he pounds the crap out of you. On your FE you get blown to bits right in the middle of your evasive move.
It's the nature of the internet beast.

You have to try to plan your evasives earlier than you expect to have them timed right on the gunner's FE.
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Lephturn - Chief Trainer
A member of The Flying Pigs
http://www.flyingpigs.com "A pig is a jolly companion, Boar, sow, barrow, or gilt --
A pig is a pal, who'll boost your morale, Though mountains may topple and tilt.
When they've blackballed, bamboozled, and burned you, When they've turned on you, Tory and Whig,
Though you may be thrown over by Tabby and Rover, You'll never go wrong with a pig, a pig,
You'll never go wrong with a pig!" -- Thomas Pynchon, "Gravity's Rainbow"