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The Security Patch updates are separate from the regular system updates. I know my S6 Edge has had one software update since the November patch yet the Android Security Patch level date is still listing as 2015-11-01.
The biggest problem with Android is that most manufacturers use a modified or a heavily modified version of Android. Extremely few devices use stock Android so users of the vast majority of devices are dependent on the device manufacturer to release the patches and updates. That is often a painfully slow process if it even happens at all. Some manufacturers of budget phones rarely if ever offer updates to their devices. It's practically unheard of if you buy an unlocked phone, especially if it's imported.
If you had a Nexus devices, the updates are likely to be done more regularly and rapidly as those devices are plain stock Android and should be getting the updates from Google directly.
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