As standard, your Sky+HD is probably set for 30 mins Instant Rewind. This means that while you are watching a channel the hard drive automatically records it and allows you to rewind up to 30 minutes (provided you have been watching that long). When you press the Record button any "previously saved" minutes you have been watching up to that point are included in the recording. When you change channel the Instant Rewind for the previous channel is cancelled and Instant Rewind for the new channel starts.
The options you have to remove this unwanted "extra" recording is to change the Instant Rewind length setting via Services / Sky+ Setup (options are Off , 5, 15, 30 or 60 minutes) otherwise just prior to the league results you can change channel and immediately change back to reset the start point of the Instant Rewind.