bramley - 5:40 am on Apr 7, 2011 (gmt 0) [edited by: bramley at 6:06 am (utc) on Apr 7, 2011]
Getting tired (sorry in UK and sunrise here) but I think the page match is done something like the way PR is distributed between links. A page titled blue and red widgets will not rank so well for blue widgets as one titled simply blue widgets. Pretty obvious really.
If you have pages called blue widgets, red widgets, black widgets; none will rank well just for widgets; for this one needs a widgets page that links out to blue widgets etc.
The nore widget of type x pages there are the more dilute each becomes for just 'widget'
A page may rank for terms other than expected because keywords used, maybe unintentionally, tilt the match towards different phrases. If that trm is not useful it is like PR leak; only ranking term leak in this case.
[edited by: bramley at 6:06 am (utc) on Apr 7, 2011]