Kind of obviouse if you look at it this way. Ak only has a set number of outs and only one wat to build a straight. Thats down. now look at any connectors in mid range- 10/9 you catch a piece of the AK and steal one of there outs, thus lessining there hand. With the 10/9 you have a wide variety of possibilities to build off of. A-10 all the way to 10-6 thats five straight possibilities with four suits. that alone is 20 outs and more if they where suited. How many will you get with AK?
now put it on a linier line
You could almost create a bell curve with this like in craps for the almost run the same. They are teaching you solid poker by playing premimum hands but sometimes the cards don't run like you want them to. Have to strattle the crown 78 is the middle of the road and can build to both side.
Now how many time you have seen them hit on the flop? to many to recount for you are focused on you two cards.
Besides playing 78 only in late postition with good pot odds