Forgery?

I went to an ice-cream restaurant to eat with my siblings. After the meal, my brother signed for his MasterCard and I exclaimed, 'Since when you change your signature?' He didn't say anything.

Did I just say that my brother signed this? How come it's my name?

My brother said he'd been doing this for a number of times already, so you can see how many people don't compare the signatures. My brother signed things like 'hamster', and many of his friends' names. So if you get a 'hamster' signature, you can call me.