If I had to guess they were using the five ask a test to a) see if it would pass a cursory inspection, which it appears it didn't, and b) refine the technique.

If I had to guess they were using the five ask a test to a) see if it would pass a cursory inspection, which it appears it didn't, and b) refine the technique. People are generally more skeptical of larger denominations (especially 100 in the US which you can sometimes get a bit of grief over when trying to use), but if someone were to notice the 5 is counterfeit somehow it's not quite as suspicious just to pay with a different (legitimate) note.