No, they all get the same OS regardless of specs. Windows is Windows regardless if it is some cheap HP Stream 11 or a Dell XPS 13. They run the same version of Windows which is my point.
And your point is based on wrong conclusions. You get the same installation disk, but the Setup process installs different drivers for each specific hardware. And there are some limitations - there is no support for old hardware and not all the functions are available on all hardware configurations. Basically each Windows installation is different. Take a hard drive from one computer and install it on another - the OS will try to identify the new hardware on first boot and install drivers needed (if available). If the hardware is totally different (AMD to Intel system, for example), the OS won't be able to boot at all. The changes are too big to fix on the fly.
Android is no different, the core OS is all the same (from Google), the drivers and skins are different. The main difference is that you (in case you make your own ROM) or the manufacturer (official releases) compile the ROM for a specific device (no other drivers needed) on a computer before it is loaded on the device. On device setup process (like Windows) is not needed. I used to port ROMs on my Galaxy S intended for different (but similar) devices by replacing the device drivers with the ones I need for my Galaxy S. And as a result 2010 Galaxy S was running Android versions that Samsung never imagined will run on this phone.
The beauty of Android is that with some knowledge you can do whatever you want with your device. Just like with your Windows computer. Don't like the UI? Change it! Missing some functionality? Add it! Want pure Vanilla Android? You have it! Want to build your own unique UI? Just do it, tools are available! No other mobile OS provides such flexibility. Want security and stability? Use original manufacturer software. In 99% it provides everything needed for an average user. There will be always people who complain about something. You can't make everyone happy. No chance.