Unfortunately there isn't a way to force the system to recognize *.mkv files as videos from the 'About This Mac' window. Macs do not natively support this file type (you'll notice that neither Quicktime nor iTunes will play them) and lack the specific decoding algorithm for the *.mkv format by default (also known as a 'codec'), so the system actually doesn't know what to do with them. Technically the *.mkv format is actually considered an open standard container format (not a video format), which means that it's not limited to any specific coding format (it can hold pretty much anything). Macs group 'container' files that aren't natively supported media types as 'Other'.
I know what you're thinking, and YES, it IS weird that it can figure it out in Finder though. I have no facts on why, only a guess. It might be because we've installed a 3rd party media player capable of handling and decoding the file. So basically the 3rd party player knows what an *.mkv file is, the system knows that the 3rd party player is in fact a video player, but unfortunately it doesn't come back around to close the information loop. It would be nice if that 3rd party player could pass that info on to the system though, wouldn't it?