Hello,
Does this make sense to you or apply to your situation:
Compatibility
GPT drives tend to include a “protective MBR.” This type of MBR says that the GPT drive has a single partition that extends across the entire drive. If you try to manage a GPT disk with an old tool that can only read MBRs, it will see a single partition that extends across the entire drive. The MBR ensures the old tools won’t mistake the GPT drive for an unpartitioned drive and overwrite its GPT data with a new MBR. In other words, the protective MBR protects the GPT data from being overwritten.
Windows can only boot from GPT on UEFI-based computers running 64-bit versions of Windows 8.1, 8, 7, Vista, and corresponding server versions. All versions of Windows 8.1, 8, 7, and Vista can read GPT drives and use them for data — they just can’t boot from them.
Other modern operating systems can also use GPT. Linux has built-in support for GPT. Apple’s Intel Macs no longer use Apple’s APT (Apple Partition Table) scheme and use GPT instead.
from http://www.howtogeek.com/193669/whats-the-difference-between-gpt-and-mbr-when-partitioning-a-drive/