While the LG G3 might not receive Android 5.1, that is no reason for those currently using the device to get upset. Obviously the handset has the specs to run the new build of Android, so one theory has LG skipping ahead to Android M from the current Android 5.0 build that is powering the former flagship.
Skipping builds is nothing new to LG. After all, the LG G3 itself went from Android 4.4.2 directly to Android 5.0, skipping a couple of minor updates along the way. If this theory does pan out, LG will no doubt include all of the added features and fixes found in Android 5.1 with the Android M update.
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source: TechValue.gr (translated)