Since Southampton has been relegated to the lower division of English football and Manchester City is fighting to finish in the top four of the Premier League table, many gamblers will play Manchester City to beat Southampton in their betslip. Manchester City ended up playing a goalless draw with Southampton, which will still cut the betslip of the gamblers who played Southampton and Manchester City to play over 1.5 and 2.5 goals. I only escaped this match because when I was placing my bet, the match had already started, which prevented me from adding it to my betslip.
Luckily for you. Personally, I don't count on Manchester City since the beginning of the season and their fluctuating performance. It is a high risk to support them while they were losing or getting a draw most times. So I assume many gamblers aren't motivated to pick a match on English Premier League with Manchester City on it.
Southampton has nothing to lose, they tried their luck and succeeded, at the opposite of Manchester City which has no reason to win.