I've only gotten mice from petco twice and I wasn't really happy with them.

The rats I get from American Rodent Supply, Classic Dum's, or Rodentpro are frozen hard. The mice I had from petco were somewhat soft and thawed between the aprox 5 minute drive home.

It's possible the one you grabbed may have been on top and experienced more thawing or something, who knows. It may be just be the temperature they keep the freezer at.

I could run and get mice from the local petland and the rodents were frozen hard....but some were freezer burnt