Think of it this way:

They make the same number of straights where at least one of the hole cards are used.

All of these straights are winners for 56 and 76. So 56 and 76 are still equal at that this point.

The Aces cant make a straight because both 56 and 76 would automatically make a higher straight.

Still equal.

A tie can occurr if there is a straight on the board (this will happen an equal amount for 56 and 76). This is mutually exclusive from the scenario where one of the hole cards are used.

Straights on the board are
A2345 (both 56 and 76 win)
23456 (56 ties, 76 wins)
34567 (both tie)
45678 (both tie)
56789 (both tie)
6789T (both tie)
789TJ (both tie)
89TJQ (both tie)
9TJQK (both lose to Aces)
TJQKA (both tie)

So they both make the same amount of straights using at least one of the hole cards but 76 wins when the 23456 straight is on the board while 56 just ties.