Why larger gauge speaker wire for low frequencies?

My question is, why do some manufacturers use a larger gauge wire for the low frequencies in a bi-wire pair of speaker cables. Examples include AZ Double Barrel and Harmonic Tech Pro 9 Reference (both of which I own), and I know there are more manufacturers who also do so.

My reason for asking is that I am constructing a pair of bi-wire cables out of bulk cable that comes as a 13awg twisted pair. My options are to run two cables per side - one to LF and one to HF, which would result in a 13awg wire to each terminal; or I could instead double up the LF cable so there would be 2 cables running to the LF and one cable to MF/HF, or 10awg aggregate to LF and 13awg to MF/HF.

All the electrical references I have looked up indicate 13awg should be plenty large for a 5-foot long run at 300wpc max into 4ohms. However, if that is true, why do manufacturers like AZ use 6awg to the LF in their Double Barrel, why does Cardas use 5.5 awg in the Golden Cross, or why is the Lowes 6awg so successful? Are there other considerations that would result in better sonics using the larger gauge?