New 870 barrel 18 or 18 1/2?

[Henry 1899]

Remington 870 tactical (wording on receiver). Barrel length measures 18 1/2" by official measurement. Here's the rub...label on box says model 870 express synthetic 18" barrel. What's up with this?

[Thunder71]

18.5" by law most likely, 18" by marketing.

[nbk13nw]

Measured mine from receiver to tip at 18 on 870 tactical