Dragon miners were made extremely poorly and do not function well with variable difficulty.  I believe their default is 1024, and I'm assuming that's what you set it to on BTC Guild, which is why it is showing up properly.  I'm pretty certain you can do the same with BitMinter, the only question is if the BitMinter variable difficulty is decreasing it, causing the miner to effectively lose speed due to its complete lack of variable difficulty support.  Basically, the miner would continue to run at diff1024, but the pool is only crediting you for 700-800ish because that's what it told you to work at instead.