I found about 50 of these & have no idea of why they were designed this way 

they are inch & half. 
Any idea?

They are designed to pull the top board down as the screw enters the bottom board.   For example, drawing the deck board down tight.  They make a similar thing to tighten down floor boards, then shear off the head.

they go thru Trex and into PT framing for decking.

 So when you use it, the bottom threads will pull the board down tight and doing so will strip the wood threads in the top board, and still make  a  tight fit?

 

 

48 minutes ago, kmealy said:

They are designed to pull the top board down as the screw enters the bottom board.   For example, drawing the deck board down tight.  They make a similar thing to tighten down floor boards, then shear off the head.

The heads don't break off this one?

The threads under the head make a clean hole for the head to sit in and looks nice.  No frayed edges.

 

https://www.mcfeelys.com/screw-fastener-web-store/screws-by-application/deck-screws-fasteners/composite-material-screws.html

 

Any thing you wish to know about a screw is in the catalogue.  

I was going to look it up, but I couldn't find anything under "funny looking screws"

1 hour ago, Ron Altier said:

I was going to look it up, but I couldn't find anything under "funny looking screws"

That's funny.... I got 6.31 million results in .54 seconds!

On 1/5/2022 at 7:30 PM, HARO50 said:

That's funny.... I got 6.31 million results in .54 seconds!

I hope that was not like one of my first forays into Google search.  I looked for "spanking"  (I might have been actually looking for something else an misspelled the word).   Let's just say it was between adults and not punishing misbehaving children.

Nope. Didn't see anything off-colour. Just all kinds of wood-working, metal-working, and assorted FASTENERS! Of course, I only looked at the first page!

The variety of screws available today is amazing. But, at my age.....?

1 hour ago, Gene Howe said:

The variety of screws available today is amazing. But, at my age.....?

 

People don't use that term anymore.............................