Honestly, I don't know diddly about anything in 20 gauge, so take this with a grain of salt (or a whole bag even).

I have a 12 gauge 870 with a longer heavy 23" rifled barrel. It was marketed as being longer to take advantage of the hot and fast sabots that do their thing at about 1,900fps. The longer barrel gives the bullet more time to accelerate while that extra gun powder is burning, yielding faster velocities.

In 12 gauge, 20" slug barrels are common so I imagine that that is all that's needed for the standard rifled slugs that fly at 1,300 or 1,500. It is quite possible that a 20 gauge only requires 18.5" to make use of the smaller amount of powder in the smaller shell, but it's really just a guess based on the 12 gauge info.