Microchips are inert and don’t move themselves. The body appears to be able to move them as muscles and fibrous connective tissues move across each other, and move within the body, fat levels change etc.
Combine this more adult movement with the puppy (especially!) microchipped at under 8 weeks when its skin is actually often big enough to hold a 12 wk old!
Implanters do their best to place the microchip in the ‘pocket’ between the shoulder blades which is the recommended place, in an adult lean animal this is easy - in a puppy it certainly isn’t!
The photo to the left shows a chip being placed completely in the wrong place. It will migrate from here down the leg probably. Many non-UK vets are taught to place the chip in the left side of the neck muscles, but this hurts so the subcutaneous route is sometimes used and they migrate.