I have seen this argument before, but I'm not sure that I buy it. Even if you count every footfall, you are not magically going to somehow use, say, only your left foot. At the end of a km, you will have 600 double paces or 1199, 1200 or 1201 single paces. Well within the margin of error.

My hypothesis is this: Actually try to count every foot when you’re out. If you count only every second footfall, you can mentally go "a-one-a-two-a-three" and so on, but counting every foot, there are just too many of them. At least I get brain overload from it.

Same reason people count things out in twos. "How many shells did you collect on the beach?" "Let's see... two, four, six, eight.. I got nine shells"

They're both shortcuts to count things with half as much counting.