Theory: maybe Canon is looking at previous shot to adjust exposure in burst?
To confirm it, take a burst sequence starting from say 1/4 seconds. You should be able to tell the change in shutter speed from the sound (if you can't, record the sound and upload it).
Can you try something else? Enable dual ISO in raw video mode, so you should see interlaced stuff in LiveView too. If you do this, metering should be correct from the start.
This just gave me an idea for the other cameras (the DSLRs): first picture in a burst can be automatic, subsequent ones can be ETTR. I don't think it makes sense on EOS M, because the only way to meter here is from LiveView, so you can enable ETTR in M mode Always On and it effectively becomes an auto mode that plays by my rules. I also don't really know how to override shutter speed in a burst sequence yet (except for
this).