Learning what sack levels Standard Mail 'Marketing Parcels' <6 oz get prepared into in MailStream Plus

MailStream Plus™, all versions, all platforms
Marketing Parcels are prepared in general per Quick Service Guide (QSG) 240K, or more detailed regulations are in the Domestic Mail Manual (DMM) in 245.11.4 for Marketing Parcels < 6 oz.
(QSG and DMM references are current as-of 2016-02-15) 

A user asked if it was correct that all of the pieces in his 40,000+ piece mailing went into MNDC (mixed-NDC) sacks.  The mailing was to destinations across the country, but 90% of the mail was to the same 5-digit area.  So he expected most of the mail to be put into 5-digit sacks.

Here is the DMM 245.11.4 list of the available sack levels, and when they can be made.

11.4  Preparing Marketing Parcels (Less Than 6 Ounces) and Irregular Parcels
11.4.3 Sacking and Labeling

a. 5-digit/scheme (optional, but required for 5-digit price), see definition in 1.4n.; allowed only for mail deposited at DNDC (or ASF when claiming DNDC prices), DSCF, or DDU.

b. SCF, allowed only for mail deposited at a DSCF or a DNDC to claim SCF price; 10-pound minimum;

c. ASF (optional), allowed only for mail deposited at an ASF to claim DNDC price; 10-pound minimum;

d. NDC, allowed only for mail deposited at a DNDC to claim the NDC price; 10-pound minimum;

e. Origin NDC (required); no minimum;

f. Mixed NDC (required); no minimum;

The mailer was entering all of the mailing at his local Business Mail Unit, not at an NDC, ASF, SCF, nor DDU.  The list above shows that sacks levels a-d can only be made for mail entered at NDCs, ASFs, SCFs, or DDUs as indicated.   
And e is only if entering the mail at an Origin NDC, which he was not doing. 
So yes, in this mailing it was correct that all of the sacks were Mixed-NDC level, and not 5-digit. 

One cannot guess at the reason why the USPS would want mail entered locally to not be in finer level sacks when possible, but it may be that the USPS prefers to process them on a machine as early in the delivery stream as possible, and so unless they are entering at the destination facility they get put into either Origin NDC or Mixed-NDC sacks.  
UPDATED:  May 15, 2019