Part of the trick is that the boys are split into two parts and progress from more inside to more outside along the wheel. Since the unit "a boy" is based on real world experience where boys can be different sizes and especially since how much space a boy takes up in an image can vary depending on angle, we mentally accept that this mashup of different amounts of body parts adds up to "one boy" rather than thinking "No, that's just 85 percent of a boy and over there we have 105 percent of a boy!"

As someone else noted, we accept in one position that two partial boys each count as a whole boy and then in another position we accept that they each count as just part of a whole boy when paired differently.

(Edited in hopes of improving clarity.)