If screws etc. are to be coated, they have to cut to a slightly smaller size so that the coating takes the final dimensions up to the standard size, otherwise either they would not fit their mating parts, or the protective coating would be stripped off, defeating the purpose.

Since only Class 2A external threads have an explicit allowance for coating, other classes both internal and external should have the size limits adjusted ...
www.efunda.com/designstandards/screws/screws_intro.cfm

Since for coated threads the tolerances apply to threads before coating (unless ... (A positive fundamental deviation indicates that the size for the thread ...
www.boltscience.com/pages/screw8.htm

Three main Classes of Fit are defined for metric screw threads :
• FINE: This has a tolerance class of 5H for internal threads and 4h for external threads.
• MEDIUM: This has a tolerance class of 6H for internal threads and 6g for external threads.
• COARSE: This has a tolerance class of 7H for internal threads and 8g for external threads.

The typical designation for a thread on a drawing is as follows
• M8 - 6e..This is a M8 course male thread with a 6e external(male) tolerance (before coating)
• M8 x 1 - 6e.. This is a M8 course male fine thread with a 6e external(male) tolerance (before coating)
• M8 - 6H... This is a M8 course female thread with a 6H internal(female) tolerance (not coated)
• M8 - 6H - LH This is a Left hand M8 course female thread with a 6H internal(female) tolerance (not coated
http://www.roymech.co.uk/Useful_Tables/Drawing/draw_dim.html