Innacurate reach calculations

While I love how FGII shows reach for creatures on mouse over, it seems that the areas shown for large reaches are inaccurate compared to the d20 rules. To illustrate my point, here are the space and reach values for a gargantuan creature, both tall and long, according to the d20 rules. ((Only pay attention to the darker inner reach area, the outer ring shows reach weapon reach))

Now here's the same values applied to a creature of gargantuan size, both with a 15 foot and 20 foot reach in fantasygrounds II.

Notice how FGII shows the reaches as a perfect square? Here are the same reaches in 15 feet and 20 feet, but this time I put yellow squares where the reaches should not extend, according to the d20 rules.

Now, I would fix this myself, if I could, but near as I can tell, the addUnderlay function FG provides is unable to reproduce these areas, being limited to a square-like 'radius' for its dimensions. If I am wrong in this assumption, by all means, tell me. Otherwise, while this is easy enough to work around mentally, I'd like to see a future update of FG include support for these kinds of areas. Thank you.