"...they simply made the head and down tube longer..."
That is true of a lot on manufactures, both in the old days and now-a-days. But there are exceptions, where the top tube length increases in proportion to seat tube length. And some manufactures use steeper angles in smaller frame sizes, for the same model.
Also keep in mind different manufacturers and people measure the seat tube differently; some to the center of the top tube, some to the top of the top tube, and some to the top of the seat tube. There can be 2-3 cm differences measuring the same bike with these 3 methods.
Not to mention classified advertisers miss-measuring.