Imagine you're on a snowmachine 10 feet south of the North Pole pointing due east. I tell you to ride for 10 miles without turning. You don't need to worry about map projections or anything - just don't turn. When you stop, are you still 10 feet south of the North Pole? Clearly not, you'll be about 10 miles south of the pole.
It's exactly the same if you start in Seattle. If you drive for 10 miles without turning, when you stop you will no longer be at Seattle's latitude - you'll have gone south.
I know, but that's not what TFA demands. Going 10 miles without turning is not "traveling in a straight line"; it's at most "traveling along a great circle line"!
And even that would require an unusually well aligned steering on a snowmobile :)
Impossible to follow if you are thinking Euclidean geometry, but a great circle is a straight line in spherical geometry. The earth isn't a sphere, but it is much closer to one than it is a plane.
And if it's supposed to mean "a straight line when projected onto a map", I'd like to know: Which map projection? :)