Hi David,
Can you open one of the images (with IrfanView or similar) to see what exposure time is stored in the EXIF section?
I have made many experiments with different ways to measure the timing on PC... Something that I found is that Canon have their own understanding how long 600s are
To avoid any problems with the stacking programs and other utilities, APT was changed to rely on Canon's readings. So the exposure is stopped when the camera reports that the desired seconds are reached.