Sylvana
the Saturday extra calories is indeed why the 10% under.
The other side of it is that as you lose weight, your TDEE will drop, so having a couple of days a week where your "normal" portions are what the new normal will be make the transition easier
and if you cannot really enjoy food one day a week, what IS the point!
because of course 5:2 is for life, not just for weight loss.
I'm fasting today for the health benefits, not because I want to get much thiner ....