Solidworks Sheet Metal Flat pattern length differences

Hello All,

I have several years of Solidworks experience, doing a lot of sheet metal parts.
I also have many, many years of ProE/Creo experience as a Power user.

I recently came across a weird error in a very simple sheet metal part.
The sheet metal generated flat pattern derived from my model, is coming out short based on the final bent part.
I had to add .27 to the overall length of the flat pattern for the final part to come out correct.

My hand calculations match what the Solidworks sheet metal generated flat pattern comes out to.

The part is a simple "C" channel out of 12GA, Galvanized steel. (.108 thick)
The "C" channel is 12.00 wide, with 3.50 legs and .625 return flanges on the ends of the 3.50 long legs.
I am using an inside bend radius of R.13 on all 4 bends.
K-Factor in sheet metal of .4
We are air bending the part in a brake press.




The lengths of the bent flanges are coming out good on the bent part, but the 12.00 width is coming out way short.
By almost .38 of an inch.

By adding .27 to the 19.43 making the flat pattern length 19.70, the part comes out good.

What am I missing here? How can this be off that much? Why?

If I mess around with the K-factor in the sheet metal parameters, I can get the Flat pattern length to match what we ended up using, but The K-factor has to set to .8 which is out of the realistic expectations, isn't it?

I have modeled this part at least 6 different ways all with the same results.
What are we missing here? why is the calculated & CAD generated flat pattern length so far off from what we really need?

Thanks,
Mechman