Your interpretation is wrong, as you are placing conditions in the law which are not there (not a terribly uncommon thing for laypersons reading statutes to do).
But try this on for size, if you want to understand why the statute is purposely written without reference to the posted speed limit:
A driver is cruising along, serenely confident he's in the right while in the left (lane), because (as he smugly notes), he is driving at the posted speed limit. From behind, comes another driver, whose passage is rendered impossible or unsafe, because the guy in the left lane is going at a speed which is less than the speed limit - driver two knows this, because he looks at his speedometer and sees that it is indicating a lower speed.
Whose speedometer is correct? It doesn't matter, because they each believes (in complete good faith) that he is correct. Meanwhile, traffic begins to back up, unsafe lane changes and passing on the right occur and (as can and will happen when disorderly traffic ensues) perhaps an accident occurs further back, innocent victims of the guy who thinks it's ok to block the passing lane.
Be real - it is virtually impossible to comply with all laws in any pursuit, whether driving, flying, hunting, whatever. If you've ever been involved in legislative drafting - writing laws (hint: I have) - you'd know that there is a great deal of effort in creating statutory constructs that work, without creating other unintended consequences. You cannot simply "imply in" an additional element (like claiming "normal speed" precludes anything which exceeds the speed limit) because you feel like being Vigilante Cop.
Drivers who refuse to yield right of way to faster traffic, regardless of the speed at which they are traveling, are violating the law, and it is thus in most states (and most certainly so in Texas) - and of greatest concern, they are impeding traffic and creating a hazard - and who wants to do that? It's confounding, really, that there are so many people who think this way, and there has been a great deal of effort placed on trying to dispel this mistaken and misguided belief. In Texas, the state's Department of Transportation is now required to place signs reading "Left lane for passing only" on divided highways, in order to clarify the drivers' obligations in this matter.
Here's a nice survey of the laws of the various states on point - there a few (Alaska, Maryland, Ohio and Puerto Rico) which actually do specify "slower than the speed limit," but most follow the Uniform Vehicle Code construct, which explicitly omits reference to speed limit.
http://www.mit.edu/~jfc/right.html
Clinging to the left lane when others want to pass creates greater danger on the road - this is why the laws like the cited one exist - and one can always conjure up a justification for their choice to do so. It will, however, still be a conjured justification, and will not serve as a defense to prosecution. Most importantly, this behavior makes that person a part of the problem by creating greater danger and reducing the effective capacity of the road.
