Extract from a weights and measures guidance note. Specifically note the reference to short measures, and no reference to long measures.
"Three Packers Rules
These set out 3 rules that packers and importers must comply with:
the contents of the packages must not be less on average than the nominal quantity
the proportion of packages which are short of the stated quantity by more than a defined amount (the ‘tolerable negative error’) should be less than a specified level
no package should be short by twice the tolerable negative error
They provide protection for consumers on short measure."
From
https://www.gov.uk/government/uploa...ed_Goods__Regulations_2006_v.3_March_2015.pdf