'Zactly… The phone hardware manufacturers have little incentive to do the work to test/tweak the updated OS versions on their phones, since that prolongs the life of old hardware instead of selling a new phone. If they do finally put out an update, then the carriers drag their feet… they couldn’t possibly release a clean update without stuffing their bloatware and custom crapola in, so they may or may not ever get around to it at all.
Example: My step-son has the same model phone as I do, but on a different carrier. We noticed differences in the icons for quickly enabling/disabling GPS, WiFi, etc. Looking further, even the menus & options under Settings were different. Big can of WTF?
Sorry, I’ll get off my soapbox now.