They're not counting total time spent with each toy. They're counting the length of each instance of playing with a toy. So if the kid plays with toy A for 1 m

They're not counting total time spent with each toy. They're counting the length of each instance of playing with a toy. So if the kid plays with toy A for 1 minute, then toy B for 1 minute, then toy A for 1 minute again, then toy B for 1 minute again, the number of interest for each toy is 1, not 2.