That's not quite the actual problem here. The "on collision"-condition will select any two instances involved in a collision, not every instance. For example, if you have two sprites, "red" and "blue", and both sprites both have 10 instances, and you create the following event:
+ Blue: On collision between Blue and Red
-> Red: Flash for 1 seconds with 0.1 seconds interval
-> Blue: Flash for 1 seconds with 0.1 seconds interval
then only those instances who are currently colliding, will flash. If only one blue instance collides with one red instance, only those two will flash.
However, in event 21 of Thijsku's image you can see that not the snowman and the snowblock, but a third sprite and the snowblock are tested for collision, so only those two are picked and not a snowman. The right snowman instance has to be picked manually in addition. For example by using
++ Snowman: Pick closest to: Sightsnowpop.X, Sightsnowpop.Y
--> (paste actions from event 21 here)
but that depends on what purpose Sightsnowpop serves.