Yea, I got your pint - we all do that (LOL).
Anyway, the 500 calories matches one of the 5-2 number for the 2 dieting days (500 for women, 600 for men), the other 5 days of the week, you basically can have at it (obviously you don’t want to overdo it).
The diet is much easier to maintain than the conventional, strict, low-calorie diet that you have to do for all 7 days, every week - since you do get to eat normally most of the time. Also, the semi-fasting seems to help prevent the metabolism slowdown that you get with conventional diets - resulting in the weights of most people no longer dropping after a period of time, despite their adherence to the conventional diet.
That makes sense.
I’ve read about how your body conserves energy with too much fasting.