Auburn's population growth is also due to annexation.
far less rapidly than suburban Seattle areas like Marysville (137%)
I'm sorry, but I find that idea hilarious. 
Which idea is hilarious? Marysville is the fastest-growing city in the state in percentage terms. Granted, its 2000 base population was much lower than most - but even so, it picked up more residents than any city except Seattle and Renton. It jumped from the state's 36th largest city to its 15th.
Snohomish County (17.7%) grew at a faster rate than King, Pierce or the state as a whole.
No, no. I don’t doubt you. I’ve just lived in or near Marysville (about 15 minutes west of) for most of my life, and my dad used to work for the City of Marysville as their finance director once upon a time, and the notion that Marysville is a Seattle suburb is a strange one to me. If anything, it's a suburb (sorta, it's complicated) of Everett.
Also, on the population growth, a good chunk of that growth in Marysville was from annexations of surrounding suburban areas that were previously unincorporated. It did grow, and quite a bit, don’t get me wrong, it’s just a bit inflated.
Growth due to annexations is one thing that takes further analysis to unravel. The census bureau does not differentiate between growth in the old city area or growth in annexed areas.
It does make sense that Marysville would have seen explosive growth in the past decade, as the Seattle-Tacoma metro area raced up the I-5 corridor. Auburn is another area at the fringes of the highway system where one would have expected to see explosive growth, too.
