Fort Bend is more middle/inner suburbs of Houston, with fewer exurbs as an overall percentage, while Montgomery is outer suburbs and exurbs. Not unrelatedly, Montgomery is majority white, while Fort Bend is one of the most diverse places in the country.
Brazoria and Galveston's voting patterns are actually more unexpected going off of demographics.
I've said this before, but Montgomery County really shouldn't be considered suburban in a general sense. Actually, if you look at the most built-up parts of Harris County, they are much closer to Fort Bend than to Montgomery; many places in "outer" Harris in fact even starts to look like Montgomery.
This is within the borders of Harris County.
