Asymmetric BLAST scores

Question

[amijaci]

I have a task for my college project to group homologous proteins. Therefore, as my first step I have to compare sequences with BLAST. I did that and I've gotten asymmetric scores for a pairs of sequences. I've went through some research papers and I've found that this is a common case, but I couldn't find the explanation for it. Can somebody explain it to me?

Also, I've seen that for the further steps of clustering algorithms those scores are averaged . Could I do the same for the purposes of my project ( of grouping/clustering proteins ) ?