Hi azircon, I think this has just been bad luck. I have not looked at the stats of your team vs. that of the other team, but you can always have a bad day and just be unlucky.
Other factors that may influence this:
Your formation stat is also adjusted for the individual stats of your players for this formation. SO maybe you might have matched a club, which no to play their 4-5-1 better than you do 3-5-2.
Then of course the individual stats of you players - keep in mind that OS is a help but not decisive. Important are the individual stats. So for example if you have two players with equal stats, tend to put the one with better shot ratios into the offensive midfield and the one with better defensive values in the midfield. Also, the team player score which also has an influence on your team stat does not influence the individual Overall strength values, but it does influence your team factor.
So my advise would be to play around a little until you find your perfect setup - and do not judge from individual games too much as you have just had a perfect day - or vice versa.