DNA vs. Protein searches
The reasons for this conclusion are:
When comparing DNA sequences, we get significantly more random matches than we get with proteins.
The DNA databases are much larger, and grow faster than Protein databases. Bigger database means more random hits!
For DNA we usually use identity matrices, for protein more sensitive matrices like PAM and BLOSUM, which allow for better search results.
Evolutionarily, protein sequences tend to diverge less than the DNA encoding them.