Mismatching build numbers when flashing Stagefright update

Hey guys,

I have an unlocked & rooted Nexus 6 (T-Mo, I believe) that I'm using for Google Fi. I'm currently on the 5.1.1 update that I got pre-root, when Project Fi pushed it to me. My build number (in About phone) is LYZ28E .

Today I was attempting to flash the official Stagefright update via TWRP, however it keeps failing with the following message:

Code:

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Package expects build fingerprint of google/shamu/shamu:5.1.1/LYZ28E/1914015 : user/release-keys;
This device has Android/aosp_shamu/shamu: 5.0/LRX21M/dees_troy11191312 : userdebug/test-keys

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Again, my ROM is the stock OTA that Google pushed me. I'm not running anything custom aside from TWRP. Any clue why the installer is showing my device as 5.0/LRX21M, when it's actually 5.1.1/LYZ28E?

Thanks in advance!