It's because you have to have moving air for an AoA sensor to work. Unmatched sensors when there isn't enough air movement for proper operation doesn't mean anything.
EDIT to add:
Hm, Boeing states:
"During rotation, pitch angle is the critical parameter that ensures tail clearance. Once the airplane is airborne and at a sufficient altitude where ground effect and crosswinds do not affect the sensor reading, AOA will provide valid information."
https://www.boeing.com/commercial/aeromagazine/aero_12/attac...
[1]http://www.ecaa.gov.et/documents/20435/0/Preliminary+Report+...