Probably confused when there were no readings at the stage they would expect to see 100 knots, by the time they realised what was going on they decided to take it into the air rather than risk a high speed stop with no idea where they were in relation to V1.
Agree with the statement about an 80 kt call out in post #33, on medium jets at light weight 100 kts isn't that far off V1, particularly at some airports which were designed with turbo props in mind rather than jets.
I will admit that when I first heard of the incident I assumed it must have been the static vent covers which were left in place as normal airspeed readings would be given until the aircraft was climbing. I didn't think the crew of a heavy jet would get airborne without any speed checks.