Well, at least until the entire world uses high DPI screens. Lets see an example:
This is a “call to action” effect on a button – it pulses slightly to get attention. This has proven to be really effective at improving “conversion” (web term to “getting you to do that thing I want you to do”). But even if you are not a designer at heart its easy to see that the text in the button is pulsing at different speeds and this creates a really jarring effect.
The reason is a bit technical, but suffice to say that it is caused by low resolution compared to the size of the button: on a standard HD display (1920×1080), the text is 99 pixels wide at rest and grows to ~104 pixels at the peak of the beat – which is a very small difference and causes this jarring effect.
But high end (and very expensive) Macintosh computers come with what is called a “High DPI” display (or “HiDPI” if your l337), that have such a large resolution that the text of this button is rendered using 200 or even 300 pixels. When the button pulses in such a high resolution, the size changes are much more fluent and you don’t get that jarring effect.
I can only assume that the designer that came up with this set up was using such a high end computer and never bothered to check how it looks in what the rest of the world uses. So, designers – let this be a lesson to you all.
[update]: in the latest Stack Exchange podcast (the one sponsored by American Venture Capital Association), they are touching a bit on the “designed by Mac user” problem, though their focus is mostly about how horrible the color correction on MS-Windows is, and there isn’t really a conclusion (though one might get the inclination that they all believe the solution to be for everyone in the world to buy “decent computers” as they call them).