Could it be that Labour focussed its efforts on only the harder constituencies to swing, rather than safe seats, & so the swing to Labour was less because no swing was "generated" in the safe seats? Had they campaigned as much in all constituencies then would they have had a larger national swing (but fewer seats)? The breakdown per constituency shows that the measly 2% swing was just as measly in the constituencies they won from the Tories. Tories lost not Labour won.
https://www.theguardian.com/politics/ng-interactive/2024/jul/05/eleven-charts-that-show-how-labour-won-by-a-landslide