Why must their be more to it, really?
Microsoft probably coded GPT (for Win2K3 Server and Vista onwards) from "scratch" but imported MBR from Win2K/XP/etc "as is" and merely updated it for full compatibility. Then continued to update the Windows Preinstallation/Recovery Environment for Win7/8/10 without ever going back to change something which already works.
It's not something anyone would really be expected to notice - or even something Microsoft necessarily noticed - unless installing an MBR and a GPT with the same WinOS version on two machines side by side. Very few users indeed would experience this, especially in the modern UEFI world where >99% of Windows installs are (or at least should be) GPT-based.
The lower-resolution MBR version might've been deliberately left unaltered to maintain maximum possible compatibility with Microsoft (and non-Microsoft) MBR-based boot loaders, file systems, and partitioning tools - not to mention all the motherboard firmwares and even the drive-embedded firmwares built around them. Look how long Microsoft clung to VGA then SVGA resolutions on earlier WinOS versions, they're not interested in spending time fixing things which already work and which are only needed to work well enough to launch Windows.
It's a cosmetic and trivial detail, anyhow. It obviously seems to bother you but I doubt it has any real significance and I doubt anyone at Microsoft deems it a high enough bug/feature/issue to merit reworking (especially given how many other problems they always need to fix in Windows).
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[/Korth]