I highly doubt those shotguns have 14 inch barrels.
Why would you doubt that?
It's a direct quote from the GSA request for bids.
And the minimum barrel length mostly just applies to normal folk. Law enforcement can get stuff that's restricted for most folk, or at least, we need to get the right special permits for. If it requires custom work, what do they care? It's government money.
(may not be custom, I don't know if there are standard 14" law-enforcement barrels, there may be)
I'm sure an 18.5" barrel just would not be tacti-cool enough for our IRS lads, what with rescuing hostages, taking down cartels, you know, all that IRS action-hero stuff they do.