Does anyone have two signatures by allegedly the same person but they are very different?

I have two marriage certificates, 10 years apart. The groom has the same name and his parents have the same name on each certificate (including identical maiden names of the groom's mother). The surnames are distinctive. However, the signatures are quite different. One signature was when he was 19 and in the US for a year. It is sort of rough and sloppy. The other signature is at age 29 and is very elegant and neat.

Did culture change his signature? It got better with time. Is this identity theft? Did someone else sign his name for him?