Why is the spacing before a paragraph on a new page affected by the spacing after the last paragraph on the previous page?

I did get a quick response from the product team, and they confirm that what is operating here is the HTML paragraph auto spacing I mentioned. That it operates even across a hard page break is a bug, which they believe has been corrected by "the new layout engine" in Word 2013, but you will continue to experience it in earlier versions. You can, however, eliminate the phenomenon by checking the Compatibility Option "Don't use HTML paragraph auto spacing."

Since your version is actually 2010/2007 rather than 2013, I am going to correct the "Applies to" on your original post. While perhaps not relevant, it would be helpful if you would also correct the Windows version by editing your question.

For reference, here's what I got from the Word product team member: "This is actually a known quirk of Word's layout: when your paragraphs are next to each other, we use the space after the first one and the space before the second one to determine their distance apart (i.e. ,a paragraph with 10pt after followed by a paragraph with 18pt before are 18pts apart, not 28pts). In this case, we're applying that logic without taking into account the page break before, so the Heading 1 at the top of the second page is getting space before equivalent to the difference between the space after the previous paragraph and its own space before (e.g., if you set the space after on the previous paragraph to 18 pt, it will effectively become 6pts [24pt-18pt])."