I can second what MadPommie wrote. In fact, I suspect that the entire wrecks thing may be slightly bugged.
I'm currently writing a complete tutorial on how to create wrecks from scratch. Also, I've written a complete list of wreck. As a result, I have shot up every wreck in the game now at least twice, and in many cases much more often than two times.
Sometimes, if you shoot once, then tractor then loot in, then shoot the wreck again, you get a bit more. This has only happened to me on Armoured Transport wrecks, though.
(Warning: this rest of this post gets a bit technical!)
What happens with armoured transport wrecks is that they usually carry 9 turrets. But thanks to the behaviour defined in fuse_solar_suprise.ini, the wreck will only drop 6 of its turrets, since the drop loot behaviour is only defined for turret hardpoints 1-6 (HpTurret01 to HpTurret06). This makes the behaviour of the other 3 turrets to be undefined (HpTurret07, HpTurret08, and HpTurret09).
The vast majority of the time you shoot up the armoured transport wreck again, nothing happens. However, once in a very rare while, the 3 remaning turrets turrets pop out on your second shot! I've tried to reproduce this effect, but without much success, which suggests to me that it's a timing issue.
On a related note, according to fuse_suprise_solar.ini, all wrecks are supposed to be indestructible (a flag is set to on in the file). However, there have been the rare occasions where the wreck does blow up when I totally reduce its health bar! Even more intriguging, any time I get more loot on my second shot, the wreck ends invariably being destructible. The interesting thing about this is that the wrecks are initially defined as destroyable in solararch.ini, but this behaviour is overridden by fuse_suprise_solar.ini's flag. This further suggests to me that there is a race issue in the code.
Only conclusion I can draw right now is that wrecks have some minor bug issues to deal with.
-- The Haen.