You're right that I haven't provided the amount of miles before or after, but here's the problem that I see:

The upgrade adjustment took place on 16th April. This shows that my parents were 4254 miles over the threshold (in this case to reach silver). The system resets their status miles to zero and then adds the 4254 as their carry-over. All of these miles were due to their long flight from Sydney to Vancouver.

However, in between there was a flight from Vancouver to Toronto of 2079 miles. The status element from these miles have been scrubbed out.

At the end of the day it doesn't really matter, as they have the right number of spending miles, they have the right status and they are unlikely to make enough extra flights to acheive gold, or requalify for silver. However, the system has ended them on 8095 status miles instead of 10174.