I would not say that I am an expert in this so my point may not be valid.

I quickly readed on Wikipedia

On modern mobile devices such as smartphones, an over-the-air update may refer simply to a software update that is distributed over Wi-Fi or mobile broadband using a function built into the operating system

From this quote we may note that the update is distributed over WiFi or mobile broadband, which not always may be available at the same time, eg. If I live in place where there is no WiFi modules, or have low signal strength, then there is possibility that I will miss the moment in which the updates are sent, and then it will be downloaded after a while.

Updates carried out by built-in manager

Some built in updating program may have settings, which allow you to customize when you want to check and download for OTA updates. Your settings may be set to check updates when device is turned on, twice per day, daily, weekly, monthly or even manually. If some devices have different settings then they may receive the updates at different time.

Updates carried out by producer

In this case the updates are carried by producer of firmware, in this case google. As already mentioned you may not be able to receive information that new update is available, because for e.g. dead battery, no signal, etc.

The updates are tend to be made when the user wont use his phone, so it do not stop user from doing whatever he/she is doing at the moment, so the update may be done at night.

Hardware difference between two the same phone models

(NOTE: this point may not be valid for some devices, including yours, as some devices may be exactly the same in first and its final release)

Sometimes even the same model of device may be a little bit different. For example at beginning xbox 360 did not had HDMI output, but after a while it was added and this probably required some other software to be made as well. Even that both types of xbox are different the model name is still the same. This make some devices with the same model name not be actually be the same inside.

There is one major difference between eg. windows 10 64 bit, and android. The difference is that the 64 bit version of windows will run on CPU which support long mode (64 bit mode) and instruction set in it will be the same in AMD and Intel. The problem with android is that different devices may have different processors with different instruction set, and this causes that phones may even require totally different code. This is causing that the updates for different devices may come in different times.

For example of this could be if you would have few WiFi cards. Lets say that you installed driver for one of them. There is very likely possibility that other cards wont run with this driver, so you would need to install different driver, which will definitely have different code. Now if the producer of this WiFi cards will find bug in all of those drivers, it is only possible to fix it one by one, or have lots of people working at the same time on different versions. However even if multiple people are working on multiple files at the same time, one may finish fix earlier than the other. In this case the update will be released quicker than for the other one.

Note that the producer could also wait for every version of code to be fixed then make update on all devices at the same time, but updating bugs even on one device decreases amount of people who would be vulnerable for this bug.

Also note that the drivers in windows are installed by you, most likely using .exe installers. In android all of those drivers are built-in, and as I mentioned before if you need different version of driver for different phones then you will spend some time on fixing all those drivers. I guess that the way google works with the update is that they are updating phones as fast as possible after they got fix for it.

Also you mentioned that even if you click manually 'check for update' button, your device says that 'system is up to date'. This may be caused because of unavailability of updating service, which is really similar to updates carried out by producer.