There are a number of different factors that go into a betting odd being calculated. These include the recent forms of teams involved, the venue where the soccer match is being played, and even the weather on the day the match is being played.
Out of all these factors, it is almost always the team's form that takes priority. Past form gives players the best indication of how the team will perform on the day of the match. The venue is an important factor, as this is when the “home advantage” can come into play. As for the weather, this is a factor as poor weather can lead to a match being postponed, or even canceled. All of these factors are taken into account when calculating the odds for a soccer match.