Since the formula is alone in its paragraph (indeed, in the entire cell), it will be rendered the right way.

The middle cell aligns its text at the centre, and here you put the formula. Now, it seems that the only reasonable way to number an equation is to use a 3×1 table with a total width of 100 % of the page, and individual cell widths of 10 %, 80 %, and 10 % (say).

This simple approach is not possible in Word 2007 and later, because if the formula is not alone on the paragraph, it will be shrunk to ‘inline style’. Indeed, using the Equation Editor 3.0 OLE object in Office Word 2003 and earlier versions, you could create a centred tab stop at the middle of the page, and a right-aligned tab stop at the right margin, and then you could manually number your equations. In fact, this was easier before the new formula editor was introduced in Microsoft Word 2007. For instance, Microsoft Word 2010 has no feature to number equations. However, if the formula is part of a line of regular text, this option is not available, but as I suggested above, there would hardly never be any reason to change the default formatting if you are writing a professional-looking document. Simply right-click any of the limits and choose 'Change limit positions' (or whatever the menu item may be called in the English version of Microsoft Word). If the formula is on its own line, then you can choose if you want the sum to have its limits above or to the right. Surely it wouldn't look good if the inlined sum was expanded! My point is that the default formatting is almost always what you want. However, this (nonsensical) example shows the change-of-formatting in action: For instance, it is not visible in the example below. Most often, you don't put very complex formulae 'inline', so you might not notice the difference in formatting very often. This is not a bug, but a feature, and indeed what you normally want! But the general rule is that equations that are positioned as regular 'objects' in the text are shown in small 'inline style', while equations that are put on their own lines are allowed to be shown in their full, expanded, glory.

I am not quite sure I understand what you wish to achieve.

Cannot insert equation in word 2010