@monzki
The issue has never been about resolution. I know that the leaks seem to indicate a "more tiles" option for WP 8.1 just as Windows 8.1 has. On Windows 8.1, that feature works by shrinking/scaling the tiles rather than expanding the tile area to cover more of the background. That, too, lends itself to what you're suggesting for WP 8.1.
The issue has been about the conflict between physical size of the rendered tiles and the physical size limits/guidelines that Microsoft itself imposed. Microsoft says that, regardless of pixel size or resolution, an object must appear to the user as being at 9mm square with a 2mm border. (Of course, they are free to break or change the rule.)
A 4.3" screen with a 800x480 or 1280x768 resolution is 2.21" wide (and 3.69" tall) -- in metric that's 56.19x93.66 mm. If we had six tiles across, we'd need 5 gaps between the tiles. We'd probably also want a left and right margin of at least the gap size. (The margins are currently wider than the inner-tile gaps, but let's shrink the margins for this exercise.) That means 6 tiles x 9mm + 5 gaps x 2mm + 2 margins x 2mm = 68mm. We're short of meeting the guidelines by 12mm (almost half an inch).
Addressing that shortfall means shrinking the tiles by 2mm each, bringing them down to 7x7mm. Now, the guidelines do mention a 7mm size -- twice! Specifically, they say "If space is severely constrained, you can use a minimum touch target size of 7 mm, as long as the width is much larger" and "The minimum touch target size is 7 mm. Use this for controls that are infrequently used or controls that are wide enough (greater than or equal to 15 mm), and only when too much height limits the design."
If the leaks are correct, I can imagine the WP team rationalizing that certain individual tiles are used infrequently and thus satisfy the 7mm height requirement. They could also rationalize that the qualifiers "as long as the width is much larger" and "controls that are wide enough (greater than or equal to 15 mm)" are no longer necessary or can be ignored in this one case because many users are requesting it.
When I earlier asked about "why not 4, 5, or 6 columns," I was joking. However, if we can accept very tiny tiles so that a 4.3" device can have six small tiles across the screen, that same size tile just happens to allow eight small tiles across the 6" screen of a Lumia 1520. Maybe this "more tiles" feature for Windows Phone 8.1 works just like it does for Windows 8. The "standard" number of tiles is dependent on physical screen size (not resolution) and the "more tiles" count is simply one more.