Propane weighs 4.2# per gallon (at 60*F)
30# cylinder full is 7.14 gallons (Full IS 80% of cylinder)
I have had 'meters' register 7.1, 7.3 & 7.5 when being refilled using a 'gallon meter'
Proper method is by 'weight'
Cylinder placed on scale ----set for cylinder weight (on collar----typically 25-28#) plus 30# (55#---58#)
Best method......weight (on scale) PLUS bleed screw open.
When liquid spurts done. Scale should trip
The propane guys meter is OFF.........cause the cylinder OPD should not have allowed continue filling beyond 80% (30#---7.1 gallon)
That's what the OPD is for......float raises up with liquid level and set to shut off inlet valve at 80% level.
OR
YOUR OPD IS FAULTY (my bet is his equipment. It's to his benefit)