On tick count 10, event 1 sets the green sprite y to 85 so it's overlapping the bottom purple sprite.
Then event 2 checks for collisions between each pair of green and purple sprites. First it checks if the green sprite is overlapping the bottom purple sprite (which it is) so it sets y to 45 and appends the UID. Then it checks if the green sprite is overlapping the top sprite (which it now is) so it sets y to 45 and appends the UID again.