The second experience group introduced in Generation III, and a direct
opposite to the Erratic group, the Fluctuating experience group
contains the Pokémon which grow the slowest of all, reaching level 100
with a whopping 1,640,000 experience points.
Fluctuating is the exact opposite of Erratic. Seviper's rival Zangoose is in the Erratic group. This is one reason behind its growth rate.
Bulbapedia: Fluctuating Exp
Also Zangoose and Seviper were both introduced in Gen 3, the same time as the growth rates Fluctuating and Erratic. I think GameFreak wanted to make the two Pokemon as opposite as possible, including their growth rates.