Fitness (CTL) is a rolling 42 day average of your daily TSS.
Fatigue (ATL) is a 7 day average of your TSS that accounts for the workouts you have done recently.
Form (TSB) is the balance of TSS equal to yesterday’s fitness minus yesterday’s fatigue.
When you take rest days, your CTL will always drop off by a similar amount as a proportion of your total CTL, but when you’re working with bigger numbers (ie CTL of 60 vs 20), the absolute numerical drop will be larger. I.e. 5% of 60 is more than 5% of 20, just to pick some numbers out of the air.