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?
The general power/impedance analyses, to which you refer, correctly indicate that most of what you describe is simply overkill.
In theory very low frequencies will use more of the 'center' of the wire because as the frequency increases it only uses a few microns of the outer surface of the conductor. That's also a reason why a lot of cable manufacturers just plate a copper base. Also as the gauge gets larger there is more surface area for the electrons to flow.
I remember the first time I tried Audience AU24 speaker cables in my system- I was using Revel speakers at the time.

I was stunned at the bass response and definition, combined with a fantastic top end, all communicated via a truly diminutive speaker cable.

Sold my giant Harmonic Tech Pro 9 and never looked back. Clearly, it ain't all about the gauge :)
I sold my AZ double barrel garden hoses and now run Morrow audio MA4 which is maybe 24 little 24 to 28 gauge wires. I have better coherence across the frequency range than I ever had with my AZ wires, including bass. As Danlib1 says "it aint all about the gauge"
Thanks all. I have always wanted to try the AU24, but haven't yet. I have enjoyed the AZ and HT cables that are both made from individually insulated PCOCC solid core copper wire (although I believe the AZ may have a small percentage stranded wire component).

These new cables I am putting together use stranded PCOCC wire that has been cryogenically and magnetically treated. I will try them in a straight double run bi-wire configuration so the LF and MF/HF each get the same sized cable using 13awg wire to each of the four bi-wire posts per speaker. I will be making runs of only 4 feet in length, so most of the articles I have read indicate that the 13awg should be more than sufficient for my 300wpc into 4ohms. However, the Tempo Electric article indicates a gauge of 10awg sized wire should be used for runs of "less than 8 feet" and amplifier output of "over 200wpc."