Many automated conversion tools (like OBS or MKV2MP4 ) append the new extension to the old filename by default rather than replacing it. Likely Content and Origin
The final .mp4 is the actual extension currently recognized by your operating system, which is the most widely compatible format for smartphones, smart TVs, and web browsers. XX EPi 2.mkv.mp4
The "EPi 2" suggests this is the second episode of a series or season. Many automated conversion tools (like OBS or MKV2MP4