That is surprising, so surprising that I have to assume the computer in question was running some kind of misbehaving ntpd that was skewing the clock.

A good quality quartz crystal is accurate to about 15 seconds a month. Even the cheap ones can manage 30 seconds a month.

Knowing why and how this happens should be a basic course in computer science classes.

Why would it be surprising? Manufacturers optimize for cost. The clock in your computer is the cheapest one they can find.

For a mobile phone it might make sense to check more often as they tend to either move timezones more often, or be powered off and then back on again.

It turns out that if you really want a device that can keep accurate time over a long period you must pay a significant amount of money. You would need a temperature compensated crystal (TCXO) or ovenized crystal oscillator (OCXO). Both of these would cost tens to hundreds of dollars, which is orders of magnitude more than hardware makers are willing to spend. Not to mention these kinds of accurate oscillators tend to be power hungry.

But that is missing the main point of the network time checks. Even if computers were perfect at telling time you still need to check network time for security reasons, especially if local users can change the device time. There are crypto attacks that rely on changing the time, and checking that the time matches another somewhat reliable source helps make those attacks harder to pull off.

Some mobile networks do a decent job of providing the time, some do a poor job, and some don't provide time at all (saw this when I was overseas). I would accept the time from the mobile network as a starting point, but prefer to get it from IP networks I trust, or from global positioning.