Why does Solidworks need to rebuild from scratch when editing the last feature...?

Just that really.  I know we've got Feature Lock coming soon, but my question is why an edit of the last feature in my long tree causes a complete rebuild.  Am I being naive in thinking that this is odd behaviour or am I missing something about the model-building process?  Or, worse still, have I got a settings turned on when I don't need it?

I'm not talking about the length of time for the rebuild, just the necessity of it rebuilding. Perhaps because of the complexity of this surface-modelled-then-solid part I'm just notining this more than I have in the past.  I'm on 2011 SP4.0.

Cheers, Keith

SolidworksParts And Features