Nickels That Appear To Be Thinner Than Regular Nickels

I'm sure it was some advice I read on one of the forums here - that is when looking at a roll of nickels to hold the roll of coins in the palm of your hands, roll the nickels and look at the rims. I have started doing that and have a question. Unfortunately, I do not have a camera, cell phone or scanner to submit pictures.

I notice coins that are thinner than others. These coins tend not to look round but slightly oval shape. They weigh the same as a regular nickel. If they are not round, how can you measure for that? what causes it? and do they have any additional value?