Dont trust the duration figure. I recall that the N900 flash lighted up twice, although I had the shutter button depressed half-way through before starting the measurement and despite having selected "always" rather than "red-eye" setting.

The documentation sais that argyll records many individual measurements for as long as the button is pressed. You are supposed to press the button, then fire the flash, and then release the button. Afterwards the software looks through the recorded data to find the flash, and discards the rest.

In normal patch-read mode (for printer profiles), the instrument can do 200 samples per second. For lack of better information, I guess that a similar rate was used.

I dont know how argyll handles two light peaks. It could have measured the whole time covered by the two events, or concatenated them, or discarded one of them, or something else. I used version 1.8.3 if you want to dig into the source code and find out.

You could also video a N900 flash and analize the time sequence with virtualdub. I guess that it is easy to figure out where this 300+ ms duration comes from, and maybe also how to avoid it for future measurements.