First of all, for the most part there are 2 seperate types of limbs out there, laminated limbs and solid glass limbs. It has been shown that laminated style limbs typically can endure more stress and preload than other types of limbs.

Let's now define preload. Preload is the amount of static load on a limb, before the bow is drawn. If you want to make a bow really really fast, then lets take your 60lb brand X bow, increase load on limbs by shortening cables and string, now you have a 70lb bow that is slightly shorter in ATA, slightly more brace height, same cam position if proportionally the string/cam were shortened the same, and a 70lb bow that is significantly faster than other bows that match that draw weight, brace height, ATA, and cam profile. You also have a limb that has a LOT more stress applied to it, and statistically will usually fail faster than other designs with similar specs, and lower load or higher quality limbs. Some manufacturers choose to apply this preload for speed and upgrade to a higher quality limb to offset failure rates, and some manufacturers choose to apply this preload and push the line keeping the same limbs used on bows with less preload with what may potentially create a higher failure rate.

Many bow companies these days have extremely fast bows, and people think it is becuase they are genious when designing cams (some are good cam designs). Truth of it is some of those gains are coming from the cam profile, but a lot of them it is a combination of increased limb preload, and cam profile as well. Making a fast bow really isn't rocket science, it's a fine balance of good cam design, limb preload, acceptable brace height, and acceptable failure rate. Increase the preload on limbs, and the bow is faster than other bows with less preload, same cam profile, same ATA and brace height.

Make sense?

All of the manufacturers you listed have or use to have bows that featured limbs from Gordon. Typically (and I could be wrong here) Gordon has only manufactured solid glass limb blanks that are then further refined by the individual manufacturers to specific dimensions and tolerances. Typically these limbs that start from a solid glass blank aren't as durable and can't handle the same preload that say a laminated limb from Barnsdale may have. Keep in mind as well that some of these companies may have certain bows with solid glass limbs, and others models with higher quality laminated limbs as well. Martin is a good example, I believe they are all laminated limbs now even on the low end Bengals, but used to have models with the solid glass limbs. I believe Hoyt is now using an all laminated limb, High Country and Elite are also using an all laminated limb made by Dave Barnsdale.