Here is a terrifically bad image made by yours truly to visually illustrate what Digital Foundry appear to be describing.
The top image is locked 60fps. Red/blue indicating new frames. In this instance we see sixty
new frames are rendered in consistent sequence for each of the two seconds.
Middle image is locked 30fps. Red/blue again indicating new frames, same consistency, but as you can see there's only half the data by comparison.
It doesn't matter if the game is a different number to these. It could be 40fps, 50fps, 59fps, 43fps, whatever. The frame count fits into each second.
The last image is Mario Kart 8. Based on Digital Foundry's observation the 64th frame is a repeat of the previous frame. This is not technically 59fps. 59fps would be like the top two images: new alternating frames throughout the time period. This is not true for Mario Kart 8. Look at the start of the second bar, and you can see two red frames next to each other. That's what's happening. The scene is refreshing sixty times per second each time, but the 64th frame in each sequence is, for some reason, a double up. Not just a drop in frame rate, but a repeated frame.
This is what we call stutter/judder and can be quite annoying for some people. It's also unusual for a game that seems to otherwise render a full 60 individual frames in a second. So yeah, I guess it's a technical bug.
Seems to be, and yes it should be fixable.