I think it is happening when a merge happens when it shouldn't. like 1.141 --> causes ..24X where X has the displayed number 1 but the colour and behaviour of a 2.
I'm not positive on that but it feels like something along those lines.
 I just got a disc which had the number was 4 in the top half and 2 in the lower. Moving the mouse off the screen and back again fixed it. I'm now figuring it's the browser rendering that's glitching rather than game logic.