According to this theory there would be EXIT 630 on the road from Dallas to El Paso. My guess is that EXIT 10 is on the other side of the bridge going the other way. Or they had plans and lost the funds. Of course this could be true in Rhode Island or Guam. 
I’m not sure what your point is here; it’s not a theory, it’s a fact (in most states, including TX). Note that on that link, only 421 miles are actually on I-20, until I-20 ends at I-10. And, indeed, the Dallas/Ft. Worth area exit numbers on I-20 range from the 430’s to the 480’s (Exit 633 doesn’t come until you’re near Louisiana, but it does come).
The exits off I-40 in the Raleigh area are numbered in the highest 200s and low 300s – I guarantee there are not 300 exits off I-40 on its entire length. But they represent mileage from where it enters the state from Tennessee, numbering increasing from west to east. (Westbound, there’s a mileage sign near Wilmington on the coast where the road begins that shows Barstow CA as something like 4000 miles down the road).
When I go north on I-81, I often stop at the Exit 42 diner in PA, which is at about Exit 197. (They haven’t changed the business name to match the mileage-based renumbering of the exits.)
So how is the Exit 42 diner? I need a good place to take a break on my trip between upstate NY and central PA.
Good one. I just got that.
The Federal government prefers states number exits to mark off distance, but does not require it. They also have a pretty nonsensical requirement about naming the parts of the exits A, B, C, or D, depending on how you turn off (as opposed to using the compass direction, so you know where you’re going).
New York numbers most of its highway exits consecutively, especially since they started doing it long before the Federal government butted in. If a new exist is needed between two existing letters, it adds a letter (There are exits 21, 21A, and 21B) on the Thruway).
Exit 10, BTW, is just past the western end of the Tappan Zee. It can be reached from either direction; I used to stop there on my way down to Long Island when it was late and I didn’t want to finish the trip.
The system is not without its amusements. For many years, there was a sign on the Adirondack Northway saying “Exit 7 to be built” (it’s been added now).
And I-88 between Albany and Binghamton used to have no exit numbers at all. You’d look for the name of the town you wanted to visit and get off. It cause no confusion, primarily because no one ever drives I-88. In fact, when then named it after Warren Anderson, a state legislator who represented Binghamton, the joke was that they gave it the name so he would use it and double the amount of traffic.
About a year ago, CalTrans (California Dept. Of Transportation) announced that it was going to start putting up numbers on the exits off all the freeways. Over 5,800 signs were planned, but so far I haven’t noticed any in Southern California.
The north-south highways will start with 0 at the Mexican border and the east-west freeways will start at the Pacific Ocean.
Parts of Interstate 10 (Santa Monica Freeway, aka Christopher Columbus Transcontintenal Highway) have some exits numbered and have for years, but no one ever refers to them by the numbers.
Although I have never noticed the sign, supposedly the exit of I-10 (by now called the San Bernardino Freeway) that takes you to Pomona College is Exit 47 as the number 47 has a somewhat mystical existence among students of that school.