Power Curve Errors

Why 1’ to almost 30’ left part of ZwiftPower power curves are flat, and high/middle W/kg values systematically underestimated? TDZ#4 @5pm today: 20m, 5m, 1m, 30s, and 15s are 3.4, 3.8, 4.8, 5.2, and 6.0 on zwift power versus 3.4, 3.8, 5.4, 7.7, and 9.8 on fit file from Zwift??? My profile is public. I’m probably not the only one.