simonlondon - 4:03 pm on Aug 13, 2013 (gmt 0)
It is a strong correlation but I would be very surprised if that's how Google value links. A page can be modified for many reasons, not all of which would render the existing links less valuable.
I would however lean towards the theory that the material change on the page probably triggered an re-assessment of the page both on and off it, including the backlinks. This may have the same end results as JS_Harris suspicion but the theory of "devaluing the old links because the page content has changed" is a bit of far fetch.