That is pretty interesting. I have never encountered that before. Your guess is as good as mine. They could be looking at the WHOIS or considering the density of Spanish words on the page, Google might be assuming your audience would be Spanish speaking visitors and classified you as such.
Let me ask you, is this a problem for you? Who is your audience?
I wonder if these tags would influence Google any differently?
code: <META HTTP-EQUIV="Content-Language" CONTENT="EN">
<META NAME="distribution" CONTENT="Global">