A value bet is when the odds offered by a bookmaker are higher than the actual likelihood of the outcome happening. This allows you to have an edge over the bookmakers. There are a few things you can do to find value bets.
I will say that I am not aware about this term "value bet" but if it is a set as described, then I think we can find this happened where an underdog is playing with the big teams and even as football outcomes might be unpredictable but mere estimating the teams strengths via historical performance and key players of the match, it is almost (if not believable) that the big team would win the match but the bookmaker have to load the big team with high odds.
Sometimes these are formats of the bookmakers to trick bettors to think otherwise because such cases are suspicions to rig the match if it is a sport bet.