Road bike fit between brands?

So I know that fit when buying a road bike is extrememly important and that different brands fit different types of bodies. But I was comparing the geometries of different brands of road bikes around the same price range (trek, cannondale, and scott) and there wasn't any measurement that differed between the three by more than a centimeter. Does this seemingly minute difference in size make up the huge difference in fit or is there something that I'm overlooking?

What Jeff B said is good. From experiance the riding style you choose is important in picking out a bike. In general, bikes such as Cervelo, Ridley, Pinarello and Specialized tend to be more of an aggressive riding position where you are crouched down like the pros. Trek "non-Pro" labelled and Cervelo's RS bikes have a headtube that's about 3 cm taller allowing one to ride more upright and thus more comfortably.

You do want to make sure the machine is easily handled so the crouch thing mention before is a good benchmark in picking out a bike. You do want to get on a trial bike and ride it around for about 1/2 hour to see if the fit is right for you. CompetativeCyclist.com has a measuring tool that is comprehensive and will put you approximately in the right sized bike.

A single centimeter makes a difference. As part of "dialing in" a road bike, you will make adjustments smaller than that. Tiny adjustments can make a significant difference, especially on a road bike where you are likely to be riding for long distances. A half centimeter adjustment to the seatpost height, for example, can really change your comfort level on the bike.

HTH

You can analyze and analyze and analyze frame measurements all you want, but you won't get the whole picture until you actually get on the bike.

Frame height isn't the only factor anymore, as seat to handlebar length can also vary widely.

For a road bike, if you stand over the top bar you should have at least an inch clearance between your crotch and the bar. From there, sitting on the bike and leaning forward should give you a low profile, but one that is relatively comfortable.

Again, a centimeter probably won't make a difference, but make sure you try a bike at a local shop for fitment if you plan on ordering online or anything.