Yes, sometimes things are more complicated than they first seem. I ordered a driver side front floor pan for my '65 when I first bought the car. I later decided that the passenger side floor pan should be also be replaced. By that time I found a less expensive unit made out of 18 ga. instead of the 20 ga. of the first unit I bought, so I ordered it instead. Didn't look exactly the same, but it's a mostly invisible non-mechanical part, so who cares? When it arrived I was impressed with how much heavier it was than the first unit. Good, right?
The problem was that the additional stamping and contours of the first pan I bought helped make it more rigid, but I couldn't really tell that until both units were installed and I could compare their rigidity. I suspect they make the cheaper pans out of thicker steel to make up for the their less rigid design. That's my theory, anyway. I don't know if this translates in any way to other sheet metal parts, but that's my experience with my floor pans.
Both pans are good enough to serve their purpose, IMO, so I won't be changing anything. I bought the first pan - which is more visually like the original - at Mac's, as I recall. I bought the 2nd cheaper pan at a place called Classic 2 Current. The place that I've heard good things about since I bought my parts is a place called Classic Sheet Metal in North Carolina.