Running after riding a bike is different from running on fresh legs. Prior activity makes you feel heavy-legged and uncoordinated when you start running. Nobody runs as fast after a hard bike ride as he or she does in a standalone run. But some triathletes lose less running performance off the bike than others. For this reason, the best runners are not always the best triathlon runners.
Consider the examples of Greg Whiteley and Hunter Kemper. Whiteley was an outstanding college runner. At Brown University he won an NCAA Championship title and was a six-time All-American. His best 5K time was 13:26. Kemper was a solid but unexceptional college runner. At Wake Forest University he earned All-Conference status once. His best time for 5K was more than a minute slower than Whiteley's.
Both Whiteley and Kemper became professional triathletes after college. One might have expected Whiteley to be the better triathlon runner, even if Kemper was the better overall triathlete, but in fact Kemper was much stronger than Whiteley off the bike. During his short triathlon career, Whiteley seldom had the fastest run split in major races, despite always being the fastest pure runner. Kemper, who is now aiming for his third Olympics, routinely records the fastest run split in triathlons despite seldom being the fastest pure runner.
Why some triathletes run better off the bike than others is not fully understood, but it appears to have something to do with differences in how individual athletes' neuromuscular systems are wired. In a 2010 study by Australian researchers, about half of the triathlete subjects tested exhibited involuntary changes to their normal running mechanics after riding a bike. These changes reduced their running economy.
Were the triathletes who maintained their running economy off the bike more experienced or better trained? No. The difference was hardwired. This was shown in a previous study by the same researchers involving elite triathletes. All of the triathletes in that subject pool were experienced and extremely well-trained, yet almost half of them also exhibited the same economy-spoiling changes in running form after cycling.
The best triathlon runners typically run five to six percent slower over a given distance in a triathlon than they do in a running race of the same distance. It would be helpful if this figure could be held up as a universal standard. In that case you could test the disparity between, for example, your freestanding 10K time and your Olympic-distance triathlon 10K run split and know that, if the disparity was 7 percent or greater, you could adjust your training to close that gap. But, because of differences in hardwiring, there is no universal standard. Some triathletes can't come within 5 percent of their standalone run times in triathlon even with perfect training.
Nevertheless, every triathlete can and should train to make that disparity as small as possible. Ways to do that include building strength on the bike so you're less fatigued and readier to run when you get off it; focusing on run training and running races during the triathlon off-season; and including plenty of bike-run transition workouts to make running off the bike second-nature.