Why did Metal Slug: 1st Mission (NGPC) have 2 different North American cartridge labels?

One had the ESRB logo while the other mentioned that it's a Neo Geo Pocket monochrome game that is also compatible with the Color.

I have a copy of the latter from one of those video game liquidators on eBay, but I always wondered why there were 2 different cartridge labels in the wild.

It kind of reminds me of this-

ESRB wasn't included on the initial run of NGPC carts until the games started selling in major retailers.