Why is the preferred IM price one that ends in 7?

It's just like most things in internet marketing: someone once allegedly split-tested it with the menu prices in a pizza restaurant in the 1960's, and unfortunately one of the people present was a "marketer"; ever since then everyone's copied it because "it must work, otherwise people wouldn't be doing it, would they?"

Meanwhile, I've seen some recent actual testing done by my own clients and former clients, in which it was discovered that:-

(i) For a short report, $10 converted better than $7 (I thought $12 might have converted better still, but I couldn't persuade the client to test it, because he already "knew" it would be worse);

(ii) For an e-book $39 converted slightly better than $37;

(iii) For some coaching videos and a membership package $22 converted a lot better than $17.

In each case, when I say "converted better", I'm referring to "more sales", not just to "more net income" (though obviously that was also true).

But the established wisdom is for prices to end in a "7", and of course it must work, otherwise people wouldn't be doing it, would they?

My theory : Even prices like $10, $20, $500 give the impression in the customer`s mind "They make such a chunk of money on me"
Prices like $17, $29 , $299 give impression like the prices were more carefully calculated , considering cost , value , size of package etc .

But I think the real winners will be prices like $18.56 , $78.40 as they look like even more carefully calculated and people get used to 7s and 9s at the end and start seeing them as marketing trick.