Everything I've said here, including my initial comment, was just speculation. But, if I had to speculate further, I'd guess it's because spam detectors tend n

Everything I've said here, including my initial comment, was just speculation. But, if I had to speculate further, I'd guess it's because spam detectors tend not to be terribly advanced. I wouldn't expect it to consider the temporal signal of "just emailed this guy and immediately got a response back," nor would I expect it to parse the headers and check times. Honestly, you'd probably have to actually talk to somebody at Google who works on or has worked on Gmail in order to get a real answer.