Postage Due Cancellations

I picked up a box full of postage due on cover. Lots of them are like these, postage due stamp is affixed, but the cancellation does not tie to the cover. There are hundreds of covers (actually mostly partial covers, front only) and they were held together with brass push pins apparently from the receiving company. I'm assuming that was something they did to account for the cost of the postage due stamps?

Did the post office cancel them before affixing? As many as there are, that is the only logical conclusion I can make. But I'm not all that familiar with postage due processes. Most are from the Denver PO, but there are other locations as well.

Any thoughts on why the cancellations do not tie to the cover?

Yes, it was very common for post offices to use various devices to "precancel" their due stamps in advance as an internal convenience. In the cases you show, they used a double oval handstamp. Parcel post style rollers are also quite common on due stamps.

Add:
A typical use of a standard precancel device on a due stamp:


Crossed pencil lines:


A steel duplex, very unusual to have the date elements in the device for this type of use (and correct too, one day after the NY mark at the lower left!):


A double oval handstamp:


A boxed parcel post style roller cancel:


A parcel post style roller cancl with wavy lines at top and bottom:


Stamps tied to a cover are more of a collector preference than a postal necessity. No doubt the USPOD/USPS got its proper revenue in each case here.