Why does the First Class letter presort show pks/bundles in our MailStream Plus presort

We have a mailing and the tray tags generated 'pks' in the trays. We have never seen that before. Please explain why there are pks/bundles in our letter tray presort?

Looking at the .tra file -

5:T-420 PCS, 21 IN

6:T-332 PCS, 3 PKS 16.6 IN

Above Tray 5 is a T – 2 foot tray and it is a full tray with 21 inches of mail.

Tray 6, is a T – 2 foot tray but it is not full, it has 16.6 inches of mail.

According to the USPS a tray has to be 85% to 100% full to be considered a full tray. So, 17.85 inches or more is considered a Full tray.

DMM 235.1.4.b

b. A full letter tray is one in which faced, upright pieces fill the length of the tray between 85% and 100% full.

And when the tray is not full, the pieces need to be bundled –

DMM 235.6.4 -

6.4 General Preparation

Grouping, bundling, and labeling are not generally required or permitted, except bundling is required in any mailing consisting entirely of card-size pieces and for pieces in overflow and less-than-full trays, and grouping is required under 6.5.

So this is why on the tray tags, we are showing 3PKS (3 bundles) since the tray is not a full tray.

Notes, these DMM reference are current as of April 2015.

UPDATED:  April 19, 2017