It may not be so much a great guess as a statistical likelihood! In 2000, there were over 2.3 million Smiths in the United States: https://blogs.ancestry.com/cm/smith-a-short-history-of-ameri... (This number has likely grown roughly alongside our population, so let's use 2000 numbers for this.) In 2000, we had 281 million people in the US, which means 0.8% of Americans have the last name Smith.

If we assume every possible SSN could be handed out, a trailing six digit entry could have anywhere up to 999 possible SSNs, so you're firing out a shotgun with a pretty big number of potential targets each time you guess an SSN.