Rolling Head Around in Dirt

My male cat, whitey, has had an ulcer on the back of his ear for around 2 months. It is almost well. It is only a small scab now. I took him to the vet right away and the vet prescribed Panalog ointment and also gave him a shot to decrease the itching so that he would not want to scratch it, which he had been doing. The shot worked great and the ointment did also. I ran out of it a couple of weeks ago and have been using generic Neosporin, which I read online is okay to use on a cat or dog as long as it does not have pain reliever in it. It has been working great, also. However, every time he goes outside, he always comes back in with dirt all over the side of his head with the ulcer on it. I was wondering if anyone knows why a cat would do this. I remember learning in science class in elementary school that if you are out in nature and do not have any water with which to wash your hands, rub some dirt between them to kill any germs on them. Therefore, I am wondering if this is what he is doing, cleaning it, in effect, even though I wash it with Ivory bar soap and rinse it extremely well afterwards. Then I put the triple antibiotic ointment on it and he never tries to wash it off, nor does he roll his head around on the carpet or the linoleum in the kitchen, no matter how long he is inside until he goes out again. He leaves it alone, except that he licks around outside of the sore on his wet fur after I wash it, as a cat will do when he comes in from the outdoors wet. The sore stays clean until the next time he comes in from the outside. Any possible reasons why a cat would do this would be most appreciated.

Did this happen with the Panalog ointment?  If not I'd ring the vet, explain it's not quite better and see if he/she wants you to get another tube of Panalog. 

Rubbing dirt strikes me as a very flaky way to get germs off one's hands - the dirt might include dried animal faeces.