Why is it that hours after you mop your floor, someone spills orange juice on it? It hasn't been mopped for, say 3 days or even a week, and during the interim no one spilled anything. Yet the day you wash it, it gets spilled on.
The day you wash the bathroom rugs (which only happens once in a blue moon or when guests are coming) someone drops a glob of toothpaste on it?
or worse yet
The day after you put fresh clean sheets on your bed (and I admit I don't change sheets regularly) a child napping in your bed wets it? I won't name the child but it wasn't my two-year old, it was another child that stayed home from school sick.