nslookup using Google DNS resolves to internal 10. IP addresses?

I’m a fairly new network analyst, so I’m not even sure I’m asking this question the right way. We have servers that host internal resources. Some of them are publicly accessible but others are all internal. How does Google DNS (or any DNS other than our own) know about our internal 10. addresses when doing a nslookup of one of our internal servers names?

I’m not sure if this is a security risk. But couldn’t someone basically map out a lot of our internal 10. architecture? I realize that’s only if they are behind our PNAC and know what they’re looking for but it makes me uncomfortable.