It looks like it could be a bunch of mould air venting. See the dots in the other lettering? The mould had tiny holes drilled through it in those spots, to allow any air between the glass and the mould to escape - the glass can make better contact with the mould, giving you a sharper embossing. It looks like you can see a "z" or something faint there, most likely a measurement in ounces. Despite the venting, the glass just didn't make good contact, or the "oz" lettering wasn't etched deep enough into the mould.
Edit: And I love that cobalt slick!