In one case the total was £4.90 I gave them a £10 note. Quickly I added a 20p coin so they could give me £5.10 10 - 4.90 = 5.10. Why was another 20 involved?

In one case the total was £4.90 I gave them a £10 note. Quickly I added a 20p coin so they could give me £5.10 10 - 4.90 = 5.10. Why was another 20 involved? Edit: I'm guessing the total was 5.10, so you gave 10.20 to get 5.10 back.