I'm a bit confused over the construction of a vertical colinear.

The docs I've seen all are more or less the same, alternating half-wave
sections of coax or hardline.
The thing that's bending my head, is that there's a velocity factor inside
the coax, that presumably isn't there on the outside.
So, while a given section is a half wave on the outside, it's probably more
like 3/4 wave inside.

My understanding is that we are trying to get to a series of dipoles all fed
in phase, and yet this seems like it's not accomplishing that.

What am I missing?